1990 AMC 12 Problem 29
Attempt Problem 29 of the 1990 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 1990 AMC 12 solutions, or check the answer key.
All problems are used with official legal permission of the Mathematical Association of America (MAA).
29.
A subset of the integers has the property that none of its members is times another. What is the largest number of members such a subset can have?
Answer: D
Small Hint:
Group integers into chains where is not a multiple of
Big Hint:
Within each chain, alternating entries give a largest allowed selection
Solution:
Partition the integers into chains with not a multiple of In each chain, no two adjacent terms may both be selected, so a maximum selection takes alternating terms starting with Equivalently, select the integers whose exponent of is even. There are The chain argument also proves no larger selection is possible.
Thus the correct answer is D.
Problem 29 in Other Years
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