Answer:
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