2001 AMC 8 Problem 25

Attempt Problem 25 of the 2001 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 2001 AMC 8 solutions, or check the answer key.

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

25.

There are 2424 four-digit whole numbers that use each of the four digits 2,2, 4,4, 5,5, and 77 exactly once. Only one of these four-digit numbers is a multiple of another one. Which of the following is it?
57245724
72457245
72547254
74257425
75427542
Answer: D
Concepts:permutationsdivisibilitycasework
Small hint:
A multiple must be twice or three times a smaller permutation
Big hint:
Divide each answer choice by 22 or 33 when possible
Solution:
A number in the 40004000, 50005000, or 70007000 range cannot be multiplied by 22 or more and remain one of the given four-digit permutations. So the smaller number must start with 22, and the larger number must be either double or triple it.
For doubles, only answer choices ending in 44 can work. But 57242=2862\frac{5724}{2}=2862 and 72542=3627\frac{7254}{2}=3627, neither of which uses exactly the digits 2,4,5,72,4,5,7.
For triples, the possible answer choices are those divisible by 33: 7245,7425,75427245,7425,7542. Dividing gives 2415,2475,25142415,2475,2514, and only 24752475 uses exactly the required digits.
Therefore 7425=3⋅24757425=3\cdot2475 is the unique listed multiple.
Thus, D is the correct answer.
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