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