Given both pipes can fill the pool in 1 h 12 mins = 72 mins
Given one pipe can fill the pool in 2 h = 120 mins
Let the other pipe fill the pool in x mins
Pipe 1:
120 mins = 1 pool
1 min = 1/120 pool
Pipe 2:
x mins = 1 pool
1 min = 1/x pool
Both pipes need 72 mins to fill up the pool:
72 (1/x + 1/120) = 1
72(x+ 120)/120x = 1
72x + 8640 = 120x
120x - 72x = 8640
48x = 8640
x = 180 mins = 3 hours
Answer: It takes 3 hours for the other pipe to fill the pool.
