Pipe A fill a cistern in 9 hours and pipe B empty the cistern in 12 hours. How much time will be taken to fill the empty cistern, if both the pipes are opened simultaneously?

This question was previously asked in

SBI Apprentice (Held On : 17 Sep 2021 Shift 1) Memory Based Paper

- 36 hours
- 39 hours
- 42 hours
- 45 hours
- 30 hours

Option 1 : 36 hours

Let's discuss the concepts related to Time and Work and Pipe and Cistern. Explore more from Quantitative Aptitude here. Learn now!

**Given:**

Time taken by pipe A to fill the cistern = 9 hours

Time taken by pipe B to empty the cistern = 12 hours

**Formula used:**

Efficiency = Work/(Time Taken)

**Calculation:**

The total work = LCM of 9 and 12 = 36 litres

Efficiency of pipe A = (36/9) = 4 litres/hour

And, Efficiency of pipe B = (36/12) = 3 litres/hour

In 1 hour, pipe A and pipe B fill = (4 - 3) litres = 1 litres

Required time to fill 36 litres = 36 hours

**∴ In 36 hours, the cistern will be filled.**

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