SSC CHSL 2021 PYQ — Question 61

The correct answer is option 4 i.e. 24.

Given:

Pipe A can empty the tank in 36 minutes

Pipe B can empty the tank in 45 minutes

Pipe C can fill the tank in 15 minutes

Concept USed:

Total Work = Efficiency × total time

Explanation:

Let the total work done by A, B and C be LCM(15, 36, 45) = 180 units

According to the given question,

Efficiency of pipe A = $-{180 \over 36}$ = - 5 units/min    (negative sign implies that pipe is emptying the tank)

Efficiency of B = $-{180 \over 45}$ = -4 units/min

And, Efficiency of C = ${180 \over 15}$ = 12 units/min

Since, B and C are opened together for 15 minutes, work done by B and C in 15 minutes = (-4 + 12) × 15 ⇒ 8 × 15 = 120 units

Now A can empty the tank in ${120 \over 5}$ = 24 minutes.

Hence the required answer is 24 minutes.