For this question I would use the similarities between pulley A, to each pulley of the question.
1. A speed of 1750 rpm is required when the drive belt is connected to pulley B. What is the diameter of pulley B?
15inches / 1500rpm = (x) inches / 1750rpm
(x) = 15 inches * 1750 rpm / 1500rpm
(x) = 17.5 inches
2.A speed of 2000 rpm is required when the drive belt is connected to pulley C. What is the diameter of pulley C?
15inches / 1500rpm = (x) inches / 2000rpm
(x) = 15 inches * 2000 rpm / 1500rpm
(x) = 20 inches
3. A speed of 3250 rpm is required when the drive belt is connected to?
15inches / 1500rpm = (x) inches / 3250rpm
(x) = 15 inches * 3250 rpm / 1500rpm
(x) = 32.5 inches