if we convert it to minutes, one pool is drained in 360 min, while using another pump requires 720 min. the problem requires them working together, so we need to find the amount of pool drained in a unit pace, being one minute. using the first pump, in 1 minute 1/360 of the pool is drained, and 1/720 for the second pump. We can add them together, thus gaining 3/720 of the pool drained in 1 minute if they are applied together. then, you calculate 720/3 to find out how many minutes it takes for the whole pool to be drained, and you get 240
