Dave is taking a multiple choice test online. He may submit his answers several times, but he passes only if one of his submissions has every question right. Every submission must carry an answer to every question, and the only thing he learns after a submission is whether he passed.
For each question Dave has estimated the probability that each of the four responses is the correct one. Those probabilities are independent of what he writes on the other questions. Given how many submissions he is allowed, Dave picks his answers so that the probability of passing is as large as possible.
Find the probability that Dave passes when he picks his answers optimally.