Question 1159363
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He gets 7 points for each correct answer, and 2 points are deducted for each wrong answer.  So his score is some multiple of 2 less than some multiple of 7.<br>
So look for a way to make a total of 87 by subtracting a multiple of 2 from a multiple of 7.<br>
The smallest multiple of 7 greater than 87 is 91, representing 91/7 = 13 correct answers.
To get a score of 87 with 13 correct answers, the number of wrong answers had to be (91-87)/2 = 4/2 = 2.
With 13 correct answers and 2 wrong answers, the number of questions omitted on the 20 question test was 5.<br>
It should be easy to see, by looking at multiples of 7 greater than 91, that there is no other way for him to get a score of 87.<br>
ANSWER: He omitted 5 questions.<br>