Calculation of allowance for receivables

I just tried a mock exam of F3, and there was a question about allowance for receivables I failed to answer correctly. Could you, please point out where I am wrong in my arguments?

Q: At 30 June 2005, a company’s allowance for receivables was $39,000.
At 30 June 2006, trade receivables totalled $517,000 and it was decided to write of debts totalling $37,000 as irrecoverable. It was also decided to adjust the allowance for receivables to 5% of the remaining trade receivables. What expense should appear in the Statement of profit or loss for the year ended 30 June 2006?

1. Getting rid of irrecoverable debts 517,000 – 37,000 = $480,000
2. Calculating allowance for receivables (5% of the remaining trade receivables) 480,000 * 5% = $24,000
3. As the opening allowance for receivables was $39,000 (credit, I assume) and the closing allowance for receivables should be $24,000 as we estimated, then we should debit allowance for receivables for 39,000 – 24,000 = $15,000.
So, I assume, that figure, $15,000 should be credited in the I/S? Yet, the right answer was $22,000, according to the revision.

Help me, please 🙂