A new 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?

So, we can say that the new pump is 1 and the old pump is 1.5 less efficient as the new pump. With this, is we were to say two of the new pumps, it would be 5 hours. However, because one is an old pump it would be 7.5 hours

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