Question 864348
rate of filling of small pipe = x
other pipe = 2.5x =2 1/2 x= 5/2 x

Filling the pool is one job

small pipe does 1/x of the job in 1 hour
big pipe does (2/5x) of the job in 1 hour

together they do 
(1/x) +(2/5x) of the job in 1 hour

together they do 7/5x of the job in 1 hour but that is =1/15

7/5x=1/15
7*15/5 =x
21=x

small pipe will take 21 hours to fill