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

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