2010 AMC 12B Problem 16
Problem 16 of 25IntermediateNumber TheoryCounting & Probability
Positive integers and are randomly and independently selected with replacement from the set What is the probability that is divisible by
Answer choices
Show solution
Solution
Factor Since is a multiple of each of is uniform modulo
If is divisible by (probability ), the product is divisible by
If is not divisible by (probability ), we need to be divisible by Checking residues, this holds exactly when or a probability of
The total probability is
Thus, the correct answer is E.