Question

Two inlet pipes can separately fill a cistern completely in 8 hours and 10 hours, respectively. They are operated for 2 hours, after which the second pipe is closed, and an outlet pipe which can drain out water from the full cistern in 20 hours is opened. How much time will it take to fill the cistern completely from the instant of opening the outlet pipe?

Options

• 8 hours

• 8 hours 40 minutes

• 7 hours

• 7 hours 20 minutes

Answer 1

7 hours 20 minutes

Explanation:

Let 'A' and 'B' are two inlet pipes and 'C' is the outlet pipe.

'A' and 'B' fill in 2 hours = (5 + 4) × 2 = 18 litres

Remaining part = 40 – 18 = 22 litres

Now, pipe 'B' is closed and outlet pipe 'C' opened.

∴ 

∴ Time taken by A and C to fill the remaining cistern

= `22/(5 - 2) = 22/3 = 7 1/3` hours 20 minutes