2010 AMC 10B Problems/Problem 12

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The minimum possible value occurs when $20\%$ of the students who originally answered "No." answer "Yes." In this case, $x=20$

The maximum possible value occurs when $30\%$ of the students who originally answered "Yes." answer "No." and the $50\%$ of the students who originally answered "No." answer "Yes." In this case, $x=50+30=80$

Subtract $80-20$ to obtain an answer of $\boxed{\mathrm{(D)} 60}$