Question

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?

Answer 1

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

Answer 2

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