Answer: 7.5 (h)
Step-by-step explanation:
Let say
a the outflow of the first pipe
b the outflow of the second pipe
T the volume of the tank.
x the time for filling with the second pipe
two pipes can fill a tank in 3 hours: a*3+b*3=T
One pipe alone can fill the tank in 5 hours : a*5=T
3*b=T -3a ==> 3b= 5a*3a=2a
b=2a/3
b*x=T ==> 2/3*a*x=T ==> 2/3*T/5*x=T ==> 2/15*x=1
x=15/2 = 7.5 (hours)
