let.........volume of the tank = V, rate of pipe 1 = R1 and rate of pipe 2 = R2
then........ R1 = V/15 and R2=V/20
⇒15R1 = 20R2 = V
thus... R1 = 4R2/3
but. .. R = R1 + R2
⇒R = 7R2/3................ = 7V/60
since..... R = V/t........... then 7V/60 = V/t
⇒t=60/7 min
It takes 8min and 34 sec{approximately}