The ratio between the efficiency of pipe Q and R is 4:3 respectively....
Let’s assume the capacity of the tank is 480 units.
Pipe S alone can empty the completely filled tank in 30 hours.
Efficiency of pipe S = 480/30 = -16 units/hour (Here negative sign represents that the pipe is emptying the tank.)
If all the pipes P, Q, R and S are opened, then the empty tank can be filled in 48 hours.
Efficiencies of pipe P, Q, R and S together = 480/48 = 10
Efficiencies of pipe P, Q and R together = 10 - (-16) = 10 + 16 = 26 units/hour Eq.(i)
The ratio between the efficiency of pipe Q and R is 4 : 3 respectively. Eq.(ii)
The time taken by pipe Q alone to fill the entire tank is 1.5 times the time taken by pipe P alone to fill the entire tank.
So the ratio between the time taken by pipe P and Q alone to fill the tank is 2:3. As we know that time and efficiency are inversely proportional to each other. Hence the ratio between the efficiency of pipe P and Q will be 3:2 respectively. Eq.(iii)
From Eq.(ii) and Eq.(iii), the ratio between the efficiency of pipe P, Q and R is 6:4:3 respectively.
From Eq.(i), the total efficiency of pipe P, Q and R together is 26.
Efficiency of pipe Q = 26 of (4/13) = 8 units/hour
Efficiency of pipe P and R together = 26-8 = 18 units/hour
Time taken by pipe P and R together to fill 30% of the tank = (480/18) of 30%
= 8 hours
Hence, option b is the correct answer.