F6 note practice question 42

Dear Sir and Madam, I am very confused about this answer of question, shown as follows:
…………………………………………………………..ALtd…….BLtd…….CLtd……..DLtd…….ELtd
Taxable Total Profits ………………………………Nil……20,000…..83,000….96,000…375,000
corporation tax at 20%…………………………………….. 4,000
at 21%………………………………………………………………………….17,430….20,160….78,750
Less: 1/400x(300,000-83,000)………………………………………….(520)
1/400X(300,000-96,000)………………………………………………………………(510)
Corporation tax liability……………………………Nil ……4,000…..17,463…..20,550….78,750
what I calculate is that 17,430-520=16,910, but answer is 17,463, will some one be willing to help me what reason?
many thanks