1987 AMC 8 Problem 24

Attempt Problem 24 of the 1987 AMC 8 below, then check your answer against the professionally curated solution from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 1987 AMC 8 solutions, or check the answer key.

All problems are used with official legal permission of the Mathematical Association of America (MAA).

24.

A multiple choice examination consists of 2020 questions. The scoring is +5+5 for each correct answer, −2-2 for each incorrect answer, and 00 for each unanswered question. John’s score on the examination is 48.48. What is the maximum number of questions he could have answered correctly?
99
1010
1111
1212
1616
Answer: D
Concepts:linear equationoptimizationparity
Small hint:
If cc answers are correct and ww are wrong, then 5c−2w=485c - 2w = 48
Big hint:
Try the largest possible cc: 5c−485c - 48 must be a nonnegative even number 2w,2w, with c+w≤20c + w \le 20
Solution:
Let cc be the number correct and ww the number wrong, so 5c−2w=48.5c - 2w = 48. Then 2w=5c−48,2w = 5c - 48, which requires cc to be even.
Trying c=14c = 14 gives w=11,w = 11, but c+w=25>20,c + w = 25 \gt 20, too many; every larger even cc would require still more answered questions. Trying c=12c = 12 gives w=6w = 6 and c+w=18≤20,c + w = 18 \le 20, which works. So the maximum is 12.12.
Thus, the correct answer is D .
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