BPP practise kit – Daniel Francine And Gregor

This is the question no. 103 part c. I believe that they have deducted 900 -2020/21 from building society interest however shouldnt it be left alone and be dealt with 12500 deductions of PA instead because no matter what capital deduction cal are done anyway seperately so there is no risk of mixing the two ?

Please correct me if I am wrong