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.