For a)

There are ways to select exactly 3 wells from the sample of 5.

Therefore the probability is

For b)

At least 3 means 3, 4 or 5...

The probability of 4 having impurity A is

The probability of 5 having A is

The sum of these 2 probabities and the answer to a) is the answer to b)

For c), just find the probability of 0, 1 or 2 having A, or subtract the answer for b) from 1