only 1/3 of a class studied for a test. those who did study were certain to pass but those who didn't study had only a 1/4 chance of passing the test. find the probability thata student did not study for thr test if she passed it
1/3, (4/12), studied and passed
2/3 did not study ... 1/4 of them (2/12) passed
so 1/3, [(2/12) / (6/12)], who passed the test didn't study