Quantitative Aptitude :: Percentage - Discussion

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Explanation:
Given 20% student answered neither question => students who answered atleast one question = 100% - 20% = 80%

Student answered first question = 75%
Student answered second question = 55%

Follow the formula n(A?B) = n(A) + n(B) - n(A?B)
=> 80% = 75% + 55% - n(A?B)
? n(A?B) = 50%