A store has found that 10% of the items it sells are retruend after Christmas. On one day, a total of 50 items are sold by 5 clerks: Clerk A sold 5 items, Clerk B sold 20 items, and the rest were sold by the other 3 clerks.

a) what is the probability that none of the items sold by Clerk A will be returned?

ans: 0.5905

b) what is the probability that exactly 3 of the items sold by Clerk B will be returned?

ans: 0.190

c) what is the probability that between 2 and 7 (inclusively) of the items sold by the other 3 clerks will be returned?

ans: 0.727

d) what is the probability that between 3 and 6 (inclusively) of the items sold by Clerk B and that exactly 4 of the items sold by the other 3 clerks will be returned?

ans: 0.044