1959 AMC 12 Problem 29

Attempt Problem 29 of the 1959 AMC 12 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 1959 AMC 12 solutions, or check the answer key.

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

29.

On an examination of nn questions a student answers correctly 1515 of the first 20.20. Of the remaining questions he answers one third correctly. All the questions have the same credit. If the student’s mark is 50%,50\%, how many different values of nn can there be?
44
33
22
11
the problem cannot be solved
Answer: D
Concepts:percentagelinear equationdivisibility
Small hint:
Express the total number correct in terms of nn
Big hint:
Set 15+n−20315+\frac{n-20}{3} equal to n2\frac{n}{2}
Solution:
The score condition gives 15+n−203=n2. 15+\frac{n-20}{3}=\frac n2. Multiplying by 66 yields 90+2n−40=3n,90+2n-40=3n, so n=50.n=50. This value also makes the number of remaining correct answers an integer. Hence there is exactly one possible value of n.n.
Therefore, the correct answer is D.
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