Implementation shortfall

Can someone explain the delay cost calculations for Q11(end of chapter questions in the CFA book, page 333) answer is in page A-24. Also, can someone tell me if it would be worth purchasing schweser Qbank and practice exams. I dont have confidence in the stall materials(especially the problems). It was good for level 1 and 2 but for level 3 it is really bad…What does QBank contain? I have practice exams from last year but not the Qbank. thanks very much.

Delay cost = (number of shares bought now / total number of shares to be bought) * ( revised purchase price - original purchase price)