What is the probability a multiple of a certain number is selected from a certain list?
We need to count how many terms are in the list (see http://www.geogebratube.org/student/b138355#material/142299 to practice that).
We need to count how many multiples of the number are in the list (which is much harder than it may seem).
Then we need to compute the probability, which in general is the number of successes divided by the total number of outcomes.