Question

New type of pump can drain a certain pool in 5 hours. an older pump can drain the pool in 15 hours. how long will it take both pumps working together to drain the pool?

Answer 1

New pump:
5 hours = 1 pool
1 hour = 1/5 pool

Old pump:
15h = 1 pool
1h = 1/15 pool

If both work together
1h = 1/5 + 1/15 = 4/15 pool

4/15 pool = 1 hour
1/15 pool = 1/4 hour
15/15 pool = 1/4 x 15 
1 pool = 3.75 hours or 3 hours 45 mins