One pipe fills a storage pool in 20 hours, another pipe fills the same pool in 15 hours.
if a third person joins and all together it takes 6hours, how long will it take the third person to fill the pool?

X=time  of the third person it would take to fill the pool 
We can suggest this equation:
6[(1/20) + (1/15) +(1/x)]=1
6[(3x+4x+60)/60x]=1
(7x+60)/10x=1
7x+60=10x
7x-10x=-60
-3x=-60
x=-60/-3
x=20

the third person alone would fill the pool in 20 hours .