2019 AMC 10B Problem 19

Below is the professionally curated solution for Problem 19 of the 2019 AMC 10B, from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2019 AMC 10B solutions, or check the answer key.

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Concepts:prime factorizationfactor counting

Difficulty rating: 2010

19.

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?

98 98

100 100

117 117

119 119

121 121

Solution:

First, note that 100,000=2555.100,000=2^55^5.

Therefore, any element of SS must be of the form 2a5b2^a5^b with 0a0 \leq a and b5.b \leq 5.

Suppose I have distinct x,ySx,y \in S with x=2a5b,x = 2^a5^b,y=2c5d.y=2^c5^d. Then, xy=2a+c5b+d.xy = 2^{a+c}5^{b+d}. Thus, 0a+c,b+d10.0 \leq a+c,b+d \leq 10. This means that there are (10+1)(10+1)=121(10+1)(10+1)=121 divisors. However, there are some divisors that can only exist if x=y,x=y, where (a,b)=(c,d).(a,b)=(c,d).

If a+c=0,a+c=0, then a=0,c=0a=0,c=0 must be true.

If a+c=10,a+c=10, then a=5,c=5a=5,c=5 must be true.

With any other value of a+c,a+c, we can have ac.a \neq c.

Similar structure holds for b+d.b+d. Thus, if a+c,b+d{0,10},a+c,b+d \in \{0,10\}, then (a,b)=(c,d),(a,b)=(c,d), thus making x=y.x=y.

This means we have to eliminate 44 choices, leaving 1214=117.121-4=117.

Thus, the answer is C .

Problem 19 in Other Years