2010 AMC 10B Problem 18
Below is the professionally curated solution for Problem 18 of the 2010 AMC 10B, from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2010 AMC 10B solutions, or check the answer key.
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Difficulty rating: 1660
18.
Positive integers and are randomly and independently selected with replacement from the set
What is the probability that is divisible by
Solution:
Note that This means that if is divisible by the whole expression is as well.
Since is divisible by we have that is divisible by with probability
Now consider not divisible by For the expression to be divisible by we must have that is divisible by
This means that
The only possibility for this is that one of the factors is mod and the other is mod
For each of the two cases, there is a chance that each of the factors is the desired modulus, for a probability of
There are two cases, which means that this happens with a probability.
The total probability is then
Thus, E is the correct answer.
Problem 18 in Other Years
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