Grace is filling her pool in the backyard with a hose that sprays 50 gallons/hour. She waited for 3 hours but the pool wasn't full, so she decides to add another hose that sprays 70 gallons/hour, and after 2 more hours the pool is full. How much water can Grace's pool contain?
Thoughts? Step-by-step reasoning:
In the first 3 hours the pool gets 50 gallons / hour x 3 hours = 150 gallons. With the addition of the other hose, the pool was filling with a rate of 50 gallons / hour + 70 gallons / hour = 120 gallons / hour. In the next 2 hours with both hoses the pool gets 120 gallons / hour x 2 hours = 240 gallons. The pool's total capacity is 240 gallons + 150 gallons = 390 gallons.
Thus, the answer is 390.