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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Difficulty rating: 2010
19.
Let be the set of all positive integer divisors of How many numbers are the product of two distinct elements of
Solution:
First, note that
Therefore, any element of must be of the form with and
Suppose I have distinct with Then, Thus, This means that there are divisors. However, there are some divisors that can only exist if where
If then must be true.
If then must be true.
With any other value of we can have
Similar structure holds for Thus, if then thus making
This means we have to eliminate choices, leaving
Thus, the answer is C .
Problem 19 in Other Years
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