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AS level further statistics, chapter 2

A chocolate manufacturer places special tokens in 2% of the bars it produces so that each
bar contains at most one token. Anyone who collects 3 of these tokens can claim a prize.
Andreia buys a box of 40 bars of the chocolate.
(a) Find the probability that Andreia can claim a prize.
(2)
Barney intends to buy bars of the chocolate, one at a time, until he can claim a prize.
(b) Find the probability that Barney can claim a prize when he buys his 40th bar of
chocolate.
(3)
(c) Find the expected number of bars that Barney must buy to claim a prize.
(1)

Can someone explain how to do part b. I can't understand the difference between that and part a.

Thank You.

(I'm in yr12 btw so if it involves content covered in Yr13/Yr2, please specify)