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There is a probability of 0 that the student plays both string and brass.

Step-by-step explanation:

Looking at the universal set, there are 40 students at this high school, 15 of which play string, 20 play brass, and 5 play neither (probably drummers).  Note how there is no intersection of A and B, which is the probability we are looking for.

Since there is no intersection in the universal set between subset A and subset B, you can say that there is a probability of 0 since there is no overlap between the two subsets.

Cheers.

if you are saying purchasing 3 social media ads and one newspaper ad counts as one purchase then there are:

6 purchases

if you are asking how many of each then:

18 social media ads

6 newspaper ads