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imc

counting and probability intermediate

Problem

What is the probability that an integer in the set is divisible by and not divisible by ?
(A)
(B)
(C)
(D)
Solution
There are integers in the set. Since every integer is divisible by , there are integers divisible by in the set. To be divisible by both and , a number must be divisible by . Since every integer is divisible by , there are integers divisible by both and in the set. So there are integers in this set that are divisible by and not divisible by . Therefore, the desired probability is
Final answer
C