r1t+t/2.5=1, we are told it takes both 1.5 hours so
1.5r+0.6=1
1.5r=0.4
r=4/15
So for this other pipe to fill the pool by itself...
4t/15=1
4t=15
t=15/4
t=3.75 hours
check..
4t/15+t/2.5=1
(4t+6t)/15=1
10t=15
t=15/10
t=3/2
t=1.5 hours, correct (for both pipes filling the pool)