A cistern is 1 4th full Two pipes which fill the cistern in 15 minutes and 20 minutes respectively a...........

Quantitative Aptitude ( CCS) Pipes&cistern

A cistern is 1/4th full. Two pipes which fill the cistern in 15 minutes and 20 minutes respectively are opened simultaneously. After 5 minutes, a third pipe which empties the full cistern in 30 minutes is also opened. In how many minutes the cistern will be full?

6 1/2

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Explanation:

Answer & Explanation
7
Explanation:
Since 1/4th is already filled, 3/4th is to filled now.
So
(1/15 + 1/20)*(5+x) – (1/30)*x = 3/4
(7/60)*5 + (7/60 – 1/30)*x = 3/4
(5/60)*x = 2/12
Solve, x = 2 mins
So total 7 minute