Plugging in a number will work here but you have to make sure it's a number that will work "nicely" in the problem.
Also, the original problem does not give the numbers as 25% and 33% but as 1/4 and 1/3. This is important to note for two reasons: first, 33% is not the same number as 1/3 - close, but not the same. Second, the fact that the original problem is written in fractions gives you the clue as to what number to pick. You pick the common denominator - in this case, 4*3 = 12 - because then the numbers in the problem will work nicely.
N = 12
1/4, or 3, answer Yes to Q1. Therefore 9 answer No to Q1
1/3 of the 3, or 1, answers Yes to Q2. Therefore, 2 of the 3 answer No to Q2.
The number of people who answered Yes to both is 1. Therefore, the number of people who did NOT answer yes to both is 12-1 = 11.
Test N=12 in the answers and find the one that equals 11. Only E works.
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