1971 AMC 12 Problem 27

Attempt Problem 27 of the 1971 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 1971 AMC 12 solutions, or check the answer key.

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

27.

A box contains chips, each of which is red, white, or blue. The number of blue chips is at least half the number of white chips and at most one-third the number of red chips. The number which are white or blue is at least 55.55. The minimum number of red chips is:
2424
3333
4545
5454
5757
Answer: E
Concepts:inequalityoptimizationbounding to limit cases
Small hint:
Let the counts be r,w,br,w,b and translate every condition into an inequality
Big hint:
From w≤2bw\le2b and w+b≥55w+b\ge55, find the least possible integer bb
Solution:
Let the counts be r,w,b.r,w,b. The conditions give w≤2b,r≥3b,w+b≥55. \begin{gathered} w\le2b,\\ r\ge3b,\\ w+b\ge55. \end{gathered} Hence 3b≥w+b≥55,3b\ge w+b\ge55, so b≥19b\ge19 and r≥57.r\ge57. Equality is possible with (w,b,r)=(36,19,57),(w,b,r)=(36,19,57), so the minimum is 57.57.
Therefore, the correct answer is E.
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