Billy takes two tests in his probability and statistics class.
The probability that Billy would pass at least one test is 0.9. The probability that he passes both tests is 0.7. The tests are of equal difficulty (that is, the probability that Billy passes test 1 is the same as the probability that he passes test 2.) What is the conditional probability of Billy passing test 2 given the event that he passes test 1?
The answer is apparently 7/8, but I have no idea how???