A firm has three telephone numbers. They receive numbers of calls that follow Poisson distributions, the first having a mean of 8, the second 4 and the third 3 in a period of half an hour. Find the probability that
a) the second and third lines will receive a total of exactly six calls in half an hour
b) the firm will receive at least twelve calls in half an hour
c) line two will receive at most six calls one hour