Gemini 3.1 Pro Preview vs GPT-5.6 Sol for bookkeeping

**1. Work out the difference**
The bank statement closing balance is £8,412.55. 
The cash book closing balance is £8,376.55. 
The difference is **£36.00** (£8,412.55 - £8,376.55). 
*(Note: The outstanding unpresented cheque and uncredited lodgement of £120.00 perfectly cancel each other out, so they do not affect this £36.00 difference).*

**2. What kind of error is it?**
This is a **transposition error** (specifically, reversing two adjacent digits when recording a number). 

**3. The arithmetic rule that identifies this class of error at a gl…

- **Adjusted bank-statement balance:**  
  £8,412.55 − £120 unpresented cheque + £120 outstanding lodgement = **£8,412.55**

- **Difference:**  
  £8,412.55 − £8,376.55 = **£36.00**  
  Thus, if the statement is correct, the cash-book balance is £36 too low.

### Apparent type of error

£36 has the classic signature of a **digit-transposition error**. For example:

\[
£451-£415=£36
\]

A reversal of digits always produces a difference divisible by **9**:

\[
451-415=36=9\times4
\]

This is the “divisibility by 9” or “casting out nines” clue. It…