2008 AMC 12B Problem 23

Below is the professionally curated solution for Problem 23 of the 2008 AMC 12B, from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2008 AMC 12B solutions, or check the answer key.

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Concepts:logarithmfactor counting

Difficulty rating: 1860

23.

The sum of the base-1010 logarithms of the divisors of 10n10^n is 792.792. What is n?n?

1111

1212

1313

1414

1515

Solution:

The sum of the base-1010 logs of the divisors is the log of their product. A number NN with d(N)d(N) divisors has divisor product Nd(N)/2.N^{d(N)/2}.

Here N=10nN = 10^n has d(N)=(n+1)2d(N) = (n + 1)^2 divisors, so the product is (10n)(n+1)2/2(10^n)^{(n+1)^2/2} and its log is n(n+1)22=792. \frac{n(n + 1)^2}{2} = 792.

Thus n(n+1)2=1584=11144=11122,n(n + 1)^2 = 1584 = 11 \cdot 144 = 11 \cdot 12^2, giving n=11.n = 11.

Thus, the correct answer is A.

Problem 23 in Other Years