With water from one hose, a swimming pool can be filled in 8 hours. a second, larger hose used alone can gill the pool in 6 hours. how long would it take to fill the pool if both hoses were used simultaneously?

So. the smaller hose, can fill the pool in 8hrs
in 1hr then, it really has done only 1/8 of the job

if we take the filled pool to be the total job time, or "t"
then the smaller hose in 1hr has only done only 1/8 in "t" amount or  

now, the bigger hose, can do the filling in 6hrs
that means in 1hr, it has only done then 1/6 in "t" time or  

now, the whole job, is a whole, or 1

so.. let us add those rates of each  hose together, to see how long they'd take 

solve for "t"

A. is the answer in this question