Irrevocable debts

Question:
“Panther owns her own business selling Gladiator dolls to department stores. At 30 June 20X2 she had the following balances in her books:

Trade receivables $31,450
Allowance for receivables (General) (as at 1 July 20X1) $(450)

A balance of $1,000 due from Selfrodges Co is considered irrecoverable and is to be written off. Horrids Co was in financial difficulty and Panther wished to allow for 60% of their balance of $800. She also decided to make a general allowance of 10% on her remaining trade receivables. What was the allowance for receivables in her statement of financial position at 30 June 20X2?”

Answers of the book:
“General allowance =10%*((Trade receivables – irrevocable debts) – Horrids) = 10%*((31.45 – 1) – 0.8) = 2.965
Specific allowance = 0.8 * 60% = 0.48
Total allowance = 3.445”

I don’t know why they could calculate the “general allowance” by the formular above, and this is my answer:
“General allowance = 10%*((Trade receivables – irrevocable debts) – specific allowance of Horrids) = 10%*((31.45 – 1) – 0.8 * 60%) = 2.997
Specific allowance = 0.8 * 60% = 0.48
Total allowance = 3.477”

Please tell me why BPP Revision kit used this method to find out the answer, thank you so much.