[Solved] HanChewie and Luke are firing cannons at a star destroyer

1) HanChewie and Luke are firing cannons at a star destroyer.The star destroyers shields are weakened sufficiently that if three different targets are hit simultaneously

,the star destroyer will explode.suppose that the probalility Han will hit the first target is p,the probability that Chewie will hit second target is q and the probability that Luke will hit the third target is r.

a).what is the probability they will need n rounds of fire(three shots,one from each cannon, in each round) to explode the star destroyer?

b,)what is the expected mumber of times each of them will need to fire to explode the star destroyer?