Probability - a clerk controlling customers > It is known that a certain clerk checks on 15 out of 100 customers. > What is the probability that from a group of 12 customers, exactly 3 will be checked on? I haven't got a faintest idea how to solve this task - could you give me any suggestions? I tried drawing a "tree", but it wasn't too effective.

You want to select exactly 3 persons from the lot of 15 people who are being controlled by the clerk and the rest of 9 from the other 85 customers. Thus the probability is $$\frac{{15 \choose 3}{85 \choose 9}}{{100 \choose 12}}$$ The denominator shows all the possible ways of selecting 12 persons from a group of 100 people.