1974 AMC 12 Problem 26

Attempt Problem 26 of the 1974 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 1974 AMC 12 solutions, or check the answer key.

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

26.

The number of distinct positive integral divisors of (30)4(30)^4 excluding 11 and (30)4(30)^4 is
100100
125125
123123
3030
none of these
Answer: C
Concepts:prime factorizationfactor counting
Small hint:
Factor 30430^4 into powers of 2,2, 3,3, and 55
Big hint:
Choose each prime exponent independently from 00 through 4,4, then remove two divisors
Solution:
Because 304=24⋅34⋅54, 30^4=2^4\cdot3^4\cdot5^4, a divisor independently chooses each of three exponents from 0,0, 1,1, 2,2, 3,3, 4.4. There are 53=1255^3=125 divisors. Excluding 11 and 30430^4 leaves 125−2=123.125-2=123.
Therefore, the correct answer is C.
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