johnycute wrote:

This semester, each of the 90 students in a certain class took at least one course from A, B, and C. If 60 students took A, 40 students took B, 20 students took C, and 5 students took all the three, how many students took exactly two courses?

We can use the formula:

Total = A + B + C - double - 2(triple) + neither

Letting d = the number of students taking exactly two courses, we have:

90 = 60 + 40 + 20 - d - 2(5) + 0

90 = 110 - d

d = 20

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