A Girl Scout troop sells cookies from house to house. One of the parents of the girls figured out that the probability that they sell a set of packs of cookies at any house they visit is 0.4, where it is assumed that they sell exactly one set to each house that buys their cookies.
a. What is the probability that the house where they make their first sale is the fifth house they visit?
b. Given that they visited 10 houses on a particular day, what is the probability that they sold exactly 6 sets of cookie packs?
c. What is the probability that on a particular day the third set of cookie packs is sold at the seventh house that the girls visit?