2019 AMC 12B Problem 14

Attempt Problem 14 of the 2019 AMC 12B 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 2019 AMC 12B solutions, or check the answer key.

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14.

Let SS be the set of all positive integer divisors of 100,000.100{,}000. How many numbers are the product of two distinct elements of S?S?

9898

100100

117117

119119

121121

Answer: C
Concepts:factorprime factorizationcomplementary counting
Difficulty rating: 1830
Solution:

Since 100,000=2555,100{,}000=2^5\cdot5^5, every divisor is 2a5b2^a5^b with 0a,b5.0\le a,b\le5. A product of two divisors is 2x5y2^x5^y with 0x,y10,0\le x,y\le10, and every such pair (x,y)(x,y) is attainable, giving 1111=12111\cdot11=121 values.

We need two distinct divisors. A value 2x5y2^x5^y is forced to be a divisor times itself only when both xx and yy have a unique split, which happens exactly when x,y{0,10}.x,y\in\{0,10\}. Those 44 corner values (1, 210, 510, 2105101,\ 2^{10},\ 5^{10},\ 2^{10}5^{10}) cannot use two distinct divisors.

The count is 1214=117.121-4=117.

Thus, C is the correct answer.

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