ary Egan needs to drain his 21 comma 000​-gallon inground swimming pool to have it resurfaced. He uses two pumps to drain the pool. One drains 15 gallons of water a minute while the other drains 20 gallons of water a minute. If the pumps are turned on at the same time and remain on until the pool is​ drained, how long will it take for the pool to be​ drained?