Working alone, pump A can empty a pool in 4 hours. Working alone, pump B can empty the same pool in 2 hours. Working together, how many minutes will it take pump A and pump B to empty the pool?
Pick: A) 65 B) 70 C) 75 D) 80 E) 85

Rationale: "pump a can empty ( 1 / 4 ) of the pool per hour . pump b can empty ( 1 / 2 ) of the pool per hour . together the pumps can empty 1 / 4 + 1 / 2 = 3 / 4 of the pool per hour . 1 pool / ( 3 / 4 ) pool per hour = 4 / 3 hours = 80 minutes . the answer is d ."
Answer: D