A statistics question

A television repair company uses a particular spare part at a rate of four per week. Assuming that requests for this spare part occur ar random, find the probability that (a) exactly six are used in a particular week, (b) at least ten are used in a two-week period, (c) exactly six are used in each of three consecutive weeks. The manager decides to replenish the stock of this spare part to a constant level n at the start of each week. (d) Find the value of / such that, on average, the stock will be insufficient no more than once in a 52-week year. (L)