A pool can be filled by one pipe in 9 hours and by a second pipe in 18 hours. How long will it take to fill the pool using both pipes
[1 / 9 + 1 / 18] = 3 / 18 [This means that 3 / 18 of the pool will be filled in 1 hour]
So, to fill the entire pool, we simply take the reciprocal of 3/18, which is of course just 18/3=6 hours.
