Question

It takes a small pump 30 hours to empty a swimmingpool, whereas a large pump can empty the same pool in 50 hours. how long it will take to empty the pool if both pumps are open?

Answer 1

The sum of the pump capacities in pools per hour will be
1/30 + 1/50 = 8/150 = 4/75

Then the time required to empty one pool is 1/(4/75) = 75/4 = 18.75 hours

It will take 18 3/4 hours to empty the pool if both pumps are open.