Question

GMAT数学题，求解析过程

If 75 percent of a class answered the first question on a certain test correctly, 55
percent answered the second question on the test correctly, and 20 percent answered
neither of the questions correctly, what percent answered both correctly?
求步骤

Answer 1

会画维恩图的话就很简单，建议看看。
下面讲讲直接算的。
只答对first question的人占总数为A，只答对second question的人占升樱迅总数为B，都对的为C，都错的为D。
由题颂孝干得
A+C=75
B+C=55
D=20

A+B+C+D=100

前三式相加减第吵此四式就得到了答案。
所以答案是50。