If 2 inlet pipes can fill a pool in one hour and 30 minutes, and one pipe can fill the pool in two hours and 30 minutes on its own, how long would the other pipe take to fill the pool on its own?

The pipe that takes 2.5 hours to fill a pool would fill 1 pool/2.5 hours = 2/5 of a pool per hour. The two inlet pipes together would fill a pool in 1.5 hours, therefore they would fill 1 pool/1.5 hours = 2/3 of a pool in one hour. We can use the subtraction of rates. 2/3 = (rate of the other inlet pipe) + 2/5, 2/3 - 2/5 = (rate of the other inlet pipe), 10/15 - 6/15 = 4/15. The other pipe fills 4/15 of a pool per hour. Since this also means it fills 4 pools in 15 hours, we only need one pool. We divide by 4 to get 3 3/4 hours to fill one pool.