50%. students (in percent) who passed the second exam also passed the first exam.
Let's imagine that there are 100 kids in the teacher's class. We know that 40 of them passed BOTH tests, and 80 passed the second test.
Because if they weren't, they wouldn't have passed the first test and consequently wouldn't have passed both, we can be sure that the group of students who passed BOTH tests is only made up of the 80 who passed the second test.
Thus, both tests were passed by 40 of the 80 pupils who passed the second one:
40/80 = 1/2 = 50%.
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