1987 AJHSME Problems/Problem 24

Problem

A multiple choice examination consists of $20$ questions. The scoring is $+5$ for each correct answer, $-2$ for each incorrect answer, and $0$ for each unanswered question. John's score on the examination is $48$ . What is the maximum number of questions he could have answered correctly?

$\text{(A)}\ 9 \qquad \text{(B)}\ 10 \qquad \text{(C)}\ 11 \qquad \text{(D)}\ 12 \qquad \text{(E)}\ 16$

Solution

Let $c$ be the number of questions correct, $w$ be the number of questions wrong, and $b$ be the number of questions left blank. We are given that $\begin{align} c+w+b &= 20 \\ 5c-2w &= 48 \end{align}$

Adding equation $(2)$ to double equation $(1)$ , we get $7c+2b=88$

Since we want to maximize the value of $c$ , we try to find the largest multiple of $7$ less than $88$ . This is $84=7\times 12$ , so let $c=12$ . Then we have $7(12)+2b=88\Rightarrow b=2$

Finally, we have $b=20-12-2=6$ . We want $c$ , so the answer is $12$ , or $\boxed{\text{D}}$ .

(As a side note, the extra work done at the end was to make sure the given situation is possible for c=12)

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1987 AJHSME Problems/Problem 24

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