(a)[2]
Find the probability that, in a randomly selected $15$-minute interval, more than $2$ customers arrive at desk $A$.
(b)[3]
Find the probability that, in a randomly chosen $5$-minute interval, the combined number of customers arriving at both desks is less than $4$.
(c)[4]
An inspector is waiting at desk $B$. She wants to wait for long enough to be $90\%$ certain of seeing at least one customer arrive at the desk. Find the shortest time she should wait, and give your answer correct to the nearest minute.