Question

A tank is full of water. A drainpipe, which can empty the full tank in 20 minutes, is opened. 12 minutes later another pipe that can fill the empty tank in 10 minutes is opened. After how much time from the beginning, in total is the tank full again (in minutes)
1. 12 minutes
2. 24 minutes
3. 36 minutes
4. 48 minutes

Answer 1

Correct Answer - Option 2 : 24 minutes

Given:

A pipe can empty the tank in 20 minutes

Concept:

Work = Efficiency × Time

Calculation:

⇒ Before the tank is full and drainpipe can empty the tank in 20 minutes

⇒ 12 minutes of work of the drain pipe = 12/20 = 3/5 of the tank

⇒ When pipe that can fill the tank in 10 minutes opened = 1/10 - 1/20 = 1/20

⇒ When both pipes are opened then they can fill the empty tank in 20 minutes

⇒ The tank filled in = 20 × (3/5) + 12 = 24 minutes

∴ The required result will be 24 minutes.