A swimming pool is being emptied through a drain at the bottom of the pool and filled by a hose at the top. The drain can empty the pool in 4 hours and the hose can fill the pool in 6 hours. The pool holds 120 liters of water. If the pool starts filled with water, how much water will be left after 3 hours? Give me reasons, before answering the question
It takes the drain 4 hours to empty the pool, so it drains at a rate of 120 liters / 4 hours = 30 liters / hour. After 3 hours, the drain will have removed 3 * 30 = 90 liters of water from the pool. It takes the hose 6 hours to fill the pool, so it fills at a rate of 120 liters / 6 hours = 20 liters / hour. After 3 hours, the hose will have added 3 * 20 = 60 liters of water to the pool. After 3 hours, there will be 120 liters + 60 liters – 90 liters = 90 liters of water left in the pool. So the final answer is 90.