Anew type of pump can drain a certain pool in
10
hours. an older pump can drain the pool in
15
hours. how long will it take both pumps working together to drain the pool?

6 hours because:

if you divide the ten in five parts you will get 2, that means 1/5 of the pool every 2 hours so in 6 hours it would be 3/5

and if you divide the 15 in 5 parts you get 3, that means 1/5 of the pool every 3 hours so in 6 hours you wil have 2/5
 
and 3/5 + 2/5 is the whole pool

A// 6 hours