One pipe can fill the swimming pool in 6h and on other in 8h. If the first run for 2h and then the second is opened. How long before the pool is full?
in 2h the 1st pipe has filled 1/3 of the pool.
1/6+1/8 = 7/24
That's how much the two pipes together can fill in an hour.
So, to fill the remaining 2/3 of the pool takes
(2/3)/(7/24) = 16/7 hours