a) Andy bought (17 -8 = 9) fewer small sheets than Ben. Since the number of sheets bought is the same, the number of big sheets that Andy bought must be 9 more than the number of big sheets that Ben bought. Each big sheet has (60 – 25 = 35) more stickers than a small sheet. Therefore, the difference in stickers is (9 x 35 = 315).

b) If Ben gave away all his stickers from the big sheets, he will be left with (17 x 25 = 425) stickers from the small sheets. Since he had 735 fewer stickers than Andy in the end, it means Andy had (425 + 735 = 1160) stickers.

From the small sheets, Andy already has (8 x 25 = 200) stickers. So, the difference of (1160 – 200 = 960) stickers must be from the big sheets. Therefore, Andy bought (960 ÷ 60 = 16) big sheets at first.

Andy bought 16 big sheets and 8 small sheets at first, so a total of 24 sheets.