1987 AIME Problem 7

Attempt Problem 7 of the 1987 AIME 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 AIME solutions, or check the answer key.

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7.

Let [r,s][r,s] denote the least common multiple of positive integers r,r, s.s. Find the number of ordered triples (a,b,c)(a,b,c) for which [a,b]=1000,[a,b]=1000, [b,c]=2000,[b,c]=2000, and [c,a]=2000.[c,a]=2000.

Answer: 70
Concepts:least common multipleprime factorizationmultiplication principle
Difficulty rating: 2230
Small Hint:

Treat the exponents of 22 and 55 independently

Big Hint:

For each prime, translate every least common multiple into a condition on pairwise maxima

Solution:

For the exponent of 5,5, all three pairwise maxima equal 3.3. Thus at least two exponents are 3:3: there is one all-33 triple and 333\cdot3 triples with exactly two 33’s, for 1010 choices.

For the exponent of 2,2, the first pair has maximum 33 while the other two have maximum 4.4. The exponent of cc must be 4,4, and the exponents of aa and bb lie in {0,1,2,3}\{0,1,2,3\} with maximum 3,3, giving 4232=74^2-3^2=7 choices. Independence gives 107=70.10\cdot7=70.

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