Help me answer this please

QUESTION 4 a) The number of telephone calls arriving per minute of a small telephone has a Poisson distribution with average 0.65. Calculate the probability that, at least three calls arrive in a period of 3 minutes. there are between 4 and 6 (inclusive) calls arrive in a period of 240 seconds. (6 marks) b) A rare type of error in the printing of postage stamps is studied. It is found that the probability of the postage stamps which have an error is 0.002. Using an appropriate approximation, calculate the probability that not more than one postage stamp has the error in a random sample of 100 stamps. (4 marks)