Math Equation help

Question

A small water pipe takes 2.5 times longer to fill a tank than a large water pipe. When both of the pipes are open, it takes 9 hours to fill the tank. If only the small pipe is used to fill the tank, how long will it take?

Answer 1

L = time for the large pipe alone to fill the tank

5L/2 = time for the small pipe alone

(1/L) + (2/5L) = 1/9 => L=63/5 and 2.5L = 63/2