Question

A teacher gave her class two exams; 60% of the class passed the second exam, but only 48% of the class passed both exams. What percent of those who passed the second exam also passed the first exam?

Answer 1

This is an example of conditional probability because we are trying to find the probability of an event occurring GIVEN the occurrence of some other event. There is a formula for this (see image attached). 
If we follow this formula, the numerator would be the probability of (A AND B)  which in this case is "48% of the class passed BOTH exams." The denominator in the formula would be that "60% of the class passed ONLY THE SECOND exam." 
Therefore, P(A and B) = 0.48, which is 48% expressed as a decimal and P(B)= 0.60, which is 60% expressed as a decimal. Then, you can figure out the answer by dividing. 

Answer 2

we are given the probility of passing the second exam of 60% and passing the first and second exam equal to 48%. In this case, to  determine the percent of those who passed the first exam, we divide 48% by 60% using the rules of probability. The answer should be 80%