Bills problem

An accountant screens large batches of bills according to the following sampling inspection plan. She inspects 4 bills chosen at random from each batch and passes the batch, if, among the 4, none is irregular. Find the probabilty that a batch will be passed if, in fact
a. 5% of its bills are irregular
b. 20% of its bills are irregular

thanks

a)
probability,P that a bill is good is .95

then the probability we choose 4 good bills is=[.95]^4 answer
b)
probiblity a bill is good is .8
then the probability we choose 4 good bills is = [.8]^4 answer

Arthur