DeepSeek V4 Pro vs GPT-5.5 for bookkeeping

The difference is **£36.00** (£8,412.55 – £8,376.55).  

**Kind of error:** Transposition error (digits swapped).  

**Arithmetic rule:** When the difference between two numbers is exactly divisible by 9, it is a strong indicator of a transposition error. Here, £36 ÷ 9 = 4, pointing to a swap of digits that differ by 4 (in this case, 5 and 1 in £451 and £415).  

**Which side is wrong:** The cash book is wrong. The receipts for £451 and £415 have been entered in the opposite order to the bank credits.  

**How to prove it:** Check the original…

Adjusted bank statement balance:

- Bank statement closing balance: **£8,412.55**
- Less unpresented cheque: **£120.00**
- Add lodgement not yet credited: **£120.00**

These cancel, so adjusted bank balance remains **£8,412.55**.

Cash book balance: **£8,376.55**

So the difference is:

**£8,412.55 − £8,376.55 = £36.00**

The bank is **£36.00 higher** than the cash book.

This is the classic sign of a **transposition error**: **£451** and **£415** have been interchanged.

Arithmetic rule: a transposition error is usually divisible by **9**. Her…