When using the big hose and the small hose, Sam can fill her pool in 15 minutes. The small hose when used alone takes 60 minutes to fill the pool. How long would it take Sam to fill her pool using only the big hose?

Question

Don't understand how to set up equation.

Answer 1

in 15 min the small hose fills 1/4 of the pool, while the big hose fills the other 3/4

3/4 in 15 min means 20 min to fill