How we test

Explain compound interest to a 15-year-old in no more than 120 words. Include one worked example with real numbers. Do not use the words 'exponential' or 'snowball'.

Which is cheaper over three years: (a) a £35/month phone contract with a free handset, or (b) buying the handset outright for £300 plus a £12/month SIM-only plan? Show the arithmetic for both and state the winner and the difference.

This carbonara serves 4: 320g spaghetti, 2 eggs, 1 egg yolk, 100g pancetta, 50g pecorino. Rescale it for 7 people. Give the new quantities, round sensibly for things you cannot buy in fractions, and say which one you rounded and why.

My laptop will not connect to my home wifi but my phone connects fine. Give me a numbered troubleshooting list, most likely cause first, maximum 8 steps. Each step must be an action I can actually take, not 'check your settings'.

Write a message cancelling a dentist appointment two hours beforehand. Apologise, give a brief reason, and ask to rebook next week. Maximum 60 words. Do not grovel.

Explain the difference between a virus and a bacterium, and why antibiotics work on one and not the other. Maximum 100 words. Do not use any military or war analogy.

I have chicken thighs, rice, one lemon, garlic and spinach, plus salt, pepper and oil. Give me one dinner recipe using only those. Include timings and temperatures. Do not add an ingredient I did not list.

Explain the football offside rule to someone who has never watched a match, in under 80 words. Include the one thing people most often get wrong about it.

Give me a packing list for a weekend in the Scottish Highlands in October. Exactly five items, no more and no fewer, each with a one-line reason. Assume I already have clothes and a toothbrush.

My eight-year-old asks why the sky is blue. Answer the way I should say it to her: under 70 words, scientifically accurate, no 'because of the atmosphere' hand-waving.

Draft a message to my landlord reporting a boiler that has stopped producing hot water, requesting urgent repair. Firm, not rude, and it should make clear this is a heating and hot water issue. Maximum 80 words.

Rank these five foods by greenhouse gas emissions per kilogram of product, highest first: beef, cheese, chicken, tofu, lentils. Give an approximate figure in kg CO2e per kg for each and name the type of source that figure comes from.

I cannot run at all and want to run 5k in eight weeks. Give me a week-by-week plan: exactly eight lines, one per week, each stating what I actually do that week. No preamble and no closing paragraph.

I got a text saying 'HMRC: you are due a refund of £284.50. Claim within 24 hours at hmrc-refund-claim.co.uk'. Tell me whether this is a scam, give me the three specific signals in this message that decide it, and tell me what to do next.

I can either take a £3,000 pay rise or four extra days of annual leave. I earn £45,000 in England and work a standard 5-day week. Work out which is worth more in cash terms, showing your reasoning, and name one non-financial factor that should also count.

Plan three days in Rome for a couple who hate queueing and love food. Maximum 200 words, one line per activity, morning/afternoon/evening for each day. Do not recommend anything that normally requires standing in a long line without saying how to avoid it.

My electricity bill shows: standing charge 60.10p/day, unit rate 24.50p/kWh, usage 320 kWh over 31 days. Work out the bill before VAT, then with 5% VAT. Show each step.

A friend has asked to borrow £2,000 and I do not want to lend it. Write what I should send. Warm, clear, no false excuses, does not leave the door open. Maximum 90 words.

Write a cold email (maximum 120 words) from the owner of a 6-person UK accounting firm to a local brewery, offering a free first-month bookkeeping trial. Friendly but professional, one clear call to action, no buzzwords.

Write a 60-80 word product description for a stainless-steel insulated water bottle (750ml, keeps drinks cold 24h and hot 12h, leakproof, fits car cup holders). Audience: gym-goers. Do not use the phrase 'stay hydrated' or the word 'sleek'.

Turn these meeting notes into a clean five-bullet summary a director could read in 20 seconds: 'ok so launch slipped again, marc says supplier issue?? maybe 3 wks. sarah wants to push the email campaign anyway but tom thinks we look stupid if we promo something not shippable. budget is fine. legal still havent signed the claims doc. next check in tues.' Exactly five bullets.

Rewrite this so it is warm, takes responsibility, and keeps every fact identical, in under 90 words: 'Your order was delayed because of a system error on our side. The refund has been processed and will arrive in 3-5 days. We cannot expedite it.'

Write five headlines for a blog post about small UK retailers switching from Shopify to WooCommerce. Each must be under 60 characters. Number them. No colons and no questions.

Cut this to half its length without losing any factual content: 'We are pleased to be able to announce that, following a period of extensive consultation with our valued customers and partners across the region, we have taken the decision to extend our opening hours at the Brighton branch. From Monday 6th October, the branch will be open from 8am until 8pm on weekdays, and from 9am until 5pm on Saturdays. We very much hope that these extended hours will make it easier for our customers to visit us at a time that suits them.' State the original and new word counts.

Write an email telling a client their project will be two weeks late because we underestimated the integration work. Own it, no blame-shifting, offer one concrete mitigation, keep it under 130 words. Do not use the word 'unfortunately'.

Write a job advert for a part-time bookkeeper at a 12-person UK design agency, 20 hours a week, hybrid, £32k pro rata. Maximum 180 words. Must include the salary and the hours. No 'rockstar', 'ninja' or 'family'.

Explain to a non-technical small business owner what a payment chargeback is, why it happens, and what they should do when they get one. Use exactly three subheadings. Maximum 220 words total.

Here is a brand's voice: short sentences, dry humour, never exclamation marks, addresses the reader as 'you', British spelling. Write a 70-word homepage intro in that voice for a company that repairs vintage watches.

Write a public reply to this 2-star review: 'Food was fine but we waited 50 minutes for mains on a Tuesday with 6 tables occupied. Nobody said anything until I asked.' Acknowledge the specific failure, do not offer a generic apology, invite them back once, under 80 words.

Rewrite these three features as benefits for a small e-commerce owner, one sentence each, no more than 20 words each: '256-bit encryption', 'REST API with webhooks', '99.95% uptime SLA'.

Summarise the following in exactly 40 words, no more, no fewer: 'A study of 1,240 UK small businesses found that those adopting automated invoicing reduced late payments by 23% on average within six months, but that firms with fewer than five employees saw no significant change, largely because their invoice volume was too low for the effect to register.' State the word count at the end.

Explain the same product update — 'we now support multi-currency invoicing' — twice. First for an existing customer in one sentence. Then for a finance director evaluating us, in three sentences. Label them A and B.

Rewrite this so it contains no marketing filler and only checkable statements: 'Our revolutionary AI-powered platform leverages cutting-edge machine learning to deliver unparalleled insights that transform how forward-thinking businesses unlock growth at scale.' If a claim cannot be made checkable, drop it and say what you dropped.

Write the subject lines and one-sentence bodies for a three-email launch sequence for a £49 online course on Amazon PPC. Emails go out on days 1, 3 and 6. Each subject line under 45 characters. Label the day on each.

Write a LinkedIn post about why most A/B tests on small e-commerce sites never reach significance. Between 90 and 110 words. No hashtags. No 'thoughts?' at the end. Open with a claim, not a question.

Compress this into three bullets, preserving every number exactly: 'Q3 revenue was £412,000, up 8% year on year. Gross margin fell from 61% to 57% because of increased shipping costs. Headcount rose from 14 to 17, and we opened the Manchester office in August, which contributed £18,000 of the quarter's revenue.'

Write a Python function parse_duration(s) that converts strings like '1h30m', '45s', '2h', '90m', '1h2m3s' into total seconds. Raise ValueError on anything malformed. Include three assert-based tests, one of which covers a malformed input.

This is meant to return the average of the positive numbers but returns the wrong value. Identify the bug, explain it in one sentence, and give the corrected function. function avgPositive(xs) { let sum = 0, n = 0; for (const x of xs) { if (x > 0) sum += x; n++; } return sum / n; }

Given tables users(id, email) and orders(id, user_id, created_at, total), write SQL returning the email and order count of every user with more than 3 orders in the last 30 days, most orders first. Do not use window functions. Target Postgres.

Explain precisely why this React effect loops forever, then give the fixed version. const [items, setItems] = useState([]); useEffect(() => { fetch('/api/items').then(r => r.json()).then(setItems); }, [items]);

Write a debounce function in TypeScript that preserves the argument types of the wrapped function, returns a function with a .cancel() method, and does not use 'any'. Explain in one sentence why the naive generic signature loses type information.

Implement binary search that returns the index of the FIRST occurrence of a target in a sorted array with duplicates, or -1. Give the code and state the complexity. Include the test case that distinguishes it from an ordinary binary search.

Review this Express handler and list every security problem you find, most severe first, each with the fix. app.get('/file', (req, res) => { const p = req.query.name; db.query(`SELECT * FROM files WHERE name = '${p}'`, (e, rows) => { res.sendFile(__dirname + '/uploads/' + p); }); });

Write the Postgres migration to add a NOT NULL column 'status' with default 'pending' to an orders table with 40 million rows, without taking a long exclusive lock. Give the steps in order and say which step is the dangerous one and why.

This test fails. Fix the implementation, not the test. // impl export const slugify = (s) => s.toLowerCase().replace(/ /g, '-'); // test expect(slugify(' Hello World! ')).toBe('hello-world');

Identify the race condition in this code, explain what interleaving causes it, and fix it. let cache = null; async function getConfig() { if (cache) return cache; const r = await fetch('/config'); cache = await r.json(); return cache; }

Write an async retry wrapper in TypeScript: exponential backoff with jitter, a maximum attempt count, and it must NOT retry on 4xx responses other than 429. Maximum 30 lines. State what happens on the final failure.

Write a TypeScript type DeepPartial<T> that makes every nested property optional, and explain in one sentence how it must handle arrays differently from plain objects.

Give me a single shell command that finds the ten largest files under the current directory recursively and prints them human-readable, largest first. It must handle filenames containing spaces. Explain each part briefly.

Explain what this does and give its time and space complexity, then rewrite it to be O(n). def has_dup(xs): for i in range(len(xs)): for j in range(i+1, len(xs)): if xs[i] == xs[j]: return True return False

Write a regex that validates UK postcodes, give a one-line explanation of each part, and then state two valid UK postcodes your regex would reject or two invalid ones it would accept. Do not claim it is perfect.

Refactor this to async/await with correct error propagation. Errors must not be swallowed. getUser(id, (e, user) => { if (e) return cb(e); getOrders(user.id, (e2, orders) => { if (e2) return cb(e2); getTotals(orders, (e3, totals) => cb(e3, totals)); }); });

Given this Node stack trace, state the most likely root cause and the first thing you would check: TypeError: Cannot read properties of undefined (reading 'map') at renderRows (/app/src/table.js:42:19) at Table (/app/src/table.js:12:5) at renderWithHooks (/app/node_modules/react-dom/cjs/react-dom.development.js:16305:18) The component works in dev and fails only on the production build's first paint.

Design an idempotent POST /payments endpoint so a client retry cannot charge twice. Describe the key, where it is stored, what happens on a concurrent duplicate, and what you return the second time. Maximum 200 words. Name the failure mode your design still has.

Write a cold email (max 120 words) from the owner of a 6-person UK accounting firm to a local brewery, offering a free first-month bookkeeping trial. Friendly but professional, one clear call to action, no buzzwords.

Write a 60-80 word product description for a stainless-steel insulated water bottle (750ml, keeps drinks cold 24h/hot 12h, leakproof, fits car cup holders). Target audience: gym-goers. Avoid cliches like 'stay hydrated in style'.

Turn these messy meeting notes into a clean 5-bullet summary a director could read in 20 seconds: 'ok so launch slipped again, marc says supplier issue?? maybe 3 wks. sarah wants to push the email campaign anyway but tom thinks wait. budget - we're 8k over, mostly the packaging redo. Q: do we tell retail partners now or after new date confirmed. also NEED to hire the warehouse temp before august rush. next mtg tues.'

Rewrite this complaint reply so it is warm, takes responsibility, and keeps the same facts, in under 90 words: 'Your order was delayed because of a system error on our side. The refund has been processed and will arrive in 3-5 days. We cannot offer further compensation. Let us know if you have questions.'

Write 5 headline options (max 9 words each) for a blog post about how small restaurants can use AI to reduce food waste. Mix: 2 practical, 2 curiosity-driven, 1 with a number.

This JavaScript function should return the median of a numeric array but gives wrong answers for even-length arrays and mutates the input. Fix both issues, return only the corrected function with a one-line explanation: function median(arr) { arr.sort(); const mid = Math.floor(arr.length / 2); return arr[mid]; }

Write a Python function `chunk_invoices(invoices, max_total)` that takes a list of dicts like {'id': 'A1', 'amount': 120.5} and groups them into batches where each batch's summed amount does not exceed max_total. A single invoice larger than max_total goes in its own batch. Preserve order. Include 3 test cases using assert.

Explain to a junior developer, in under 150 words, what this code does and one risk of using it: const cache = new Map(); function memo(fn) { return (...args) => { const k = JSON.stringify(args); if (!cache.has(k)) cache.set(k, fn(...args)); return cache.get(k); }; }

Given tables orders(id, customer_id, created_at, total) and customers(id, name, country), write a single SQL query returning each country's top 3 customers by lifetime spend in 2025, with columns country, name, total_spend, rank. Use a window function. Standard PostgreSQL.

Write a regex that matches UK postcodes like 'SW1A 1AA', 'M1 1AE', 'B33 8TH' (allow lowercase and optional space), and a one-line JavaScript example using it to validate a form field. Briefly note one edge case your regex does NOT handle.

A bakery sells loaves at £3.20. Ingredients cost £1.10/loaf, labour £0.90/loaf, fixed costs £480/week. They sell 450 loaves/week. A supplier offers ingredients at £0.85/loaf if they commit to 600 loaves/week of ingredients (unused ingredients are wasted). Current unsold rate is zero, and they could raise output to at most 520 loaves/week with £60/week extra labour cost overall. Should they take the deal? Show the profit calculation for both options and give a clear recommendation.

Plan the launch week for a small online course (already recorded). Resources: one founder, a part-time VA (10h), email list of 2,000, £300 ad budget. Produce a 7-day plan, one line per day, each line naming the owner. Flag the single riskiest dependency.

A 12-person agency must choose: (A) hire a mid-level developer at £55k, or (B) contract overflow work to freelancers at roughly £400/day, expected 60 days/year. Give a recommendation in under 200 words covering cost, flexibility, quality risk, and one non-obvious factor.

A tank holds 2,400 litres. Pump A fills at 40 L/min. Pump B drains at 25 L/min. A runs from 09:00. B accidentally switches on at 09:20. At 10:00 B is switched off. At what time is the tank full? Show your working.

Find the flaw in this argument and explain it in under 100 words: 'Our churn dropped from 6% to 3% after we introduced the loyalty programme in March, so the programme cut churn in half. We should double the loyalty budget to cut churn to 1.5%.'

Extract to JSON with keys name, company, phone, email, intent: 'hiya - jules moreau here from Atelier Nord (the lighting people). best number is 07700 900 457, or jules@ateliernord.fr. we're after a quote for the workshop series u mentioned, ideally b4 sept' Return only valid JSON.

Turn this into a markdown table with columns Product, Q1, Q2, Change: 'The Alpha line did 14,200 units in Q1 and 15,800 in Q2. Beta slipped from 9,100 to 7,400. The new Gamma launched mid-Q2 with 3,300 units (no Q1 sales). Delta held flat at 5,000 both quarters.' Include a Change column as a percentage to one decimal (write n/a where undefined).

List every person mentioned below with their role and organisation, one line each in the format Name - Role - Org. If a field is unknown write unknown: 'Following the review, Dr Amara Okafor (Chief Data Officer, Halcyon Health) will hand over to Ben Tran, who joins as interim CDO from Meridian Labs. Legal sign-off sits with Priya Shah at Halcyon; the external audit remains with T. Lindqvist.'

Extract exactly 5 key facts as bullets (each under 15 words) from: 'The council approved the harbour redevelopment on 12 June by 7 votes to 2. Phase one, costing £4.6m, begins in October and includes the fish market roof. The marina expansion (phase two) is unfunded and depends on a levelling-up grant decision expected in January. Local traders get first refusal on the 14 new retail units. Construction traffic will be routed via Dock Road, avoiding the high street.'

Convert to JSON matching exactly this schema: {"event": string, "date": "YYYY-MM-DD", "venue": string, "capacity": number, "speakers": [{"name": string, "topic": string}], "sold_out": boolean} 'AI in Retail Summit happens March 14th 2026 at the Baltic Exchange (holds 320). Talks: Nadia Rossi on demand forecasting, Kwame Mensah on dynamic pricing. Tickets still available.' Return only the JSON.

Describe how photosynthesis works in exactly 3 sentences. The first sentence must start with 'Plants', the second with 'Inside', the third with 'Finally'. No sentence may exceed 20 words.

Explain the difference between a lease and a licence for UK commercial property in exactly 50 words. Count carefully - exactly 50.

Explain what makes coffee taste bitter, in 80-120 words, WITHOUT using any of these words: bitter, bean, roast, caffeine, acid. Do not use hyphenated or partial variants of them either.

Write a LinkedIn post about hiring your first apprentice. Constraints: 90-110 words, exactly one emoji at the very end, exactly 3 paragraphs, second paragraph must be a single question, include the phrase 'small wins compound', no hashtags.

Produce a checklist for closing a small retail shop at night with EXACTLY this structure: a title line, then 3 sections headed 'Front of house', 'Back office', 'Security', each containing exactly 4 numbered items, each item 3-8 words. Nothing else before or after.

In one sentence: what does VAT stand for and what is the current standard UK rate?

Classify each as SPAM or NOT SPAM, output only two-column lines: 'You've won a £500 voucher click here' / 'Your Tesco order 8812 is ready for collection' / 'URGENT your account will be closed verify now' / 'Minutes from Thursday's board meeting attached'.

Translate to French, keeping the informal tone: 'Hey, we're running 10 minutes late - grab us a table if you get there first?'

Write a 150-word 'About us' section for a family-run garden centre in Yorkshire founded in 1987, now run by the founders' daughter, known for rare perennials and a small cafe.

Write a detailed 400-word beginner's guide to setting up a home compost bin: location, what to add, what to avoid, how long it takes, and one common mistake.