I don’t know whose workings you have copied, but although the final answer is correct their workings are a bit odd 🙂
The allowance is currently 10 and is to be increased to 5% of receivables i.e. to 16 (5% x 320). So the cost of increasing it is 16 – 10 = 6. So the total admin cost is 325 (from the trial balance) + 6 = 331