One pipe can fill a swimming pool in 10 hours, while a second pipe can fill the pool in 6 hours. If the second pipe is turned on 2 hours after the first, how long will it take to fill up the pool?

6 4/5 hours

Step-by-step explanation:

If one pipe can fill the pool in 10 hours, then it can fill 1/5 of the pool in 2 hours.

If two pipe's can fill the pool in 6 hour's, then it can fill 1/5 of the pool 1 1/5 hours.

So:

6-1 1/5=4 4/5

4 4/5+2=6 4/5

a negative exponent is translated as follows:

so, we have

the answer to the expression is 83