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Kush mudgal

· started a discussion

· 1 Months ago

Language of question ambiguous (Question: 113283.0 )

It must be given time taken by c to nai empty it.. total time is 50.

Question:

A pipe can fill a tank in 12 minutes and another pipe is 15 minutes but a third can empty it in 6 minutes. The first two pipes are kept open for 5 minutes in the beginning and then the third pipe is also opened. In what time is the cistern emptied?

Options:
A) 40 minutes
B) 50 minutes
C) 45 minutes
D) 55 minutes
Solution:
Ans: (c) Cistern filled in 5 minutes = \(5\left ( \cfrac{1}{12}+\cfrac{1}{15} \right )=\cfrac{3}{4}\) 

Net work done by 3 pipes in 1 minute=\(\left ( \cfrac{1}{12}+\cfrac{1}{15} \right )-\cfrac{1}{6}=-\cfrac{1}{60}\).

-ve sign shows that \(\cfrac{1}{60}\) part is emptied in minute

\(\therefore\) \(\cfrac{3}{4}\) part is emptied in \(60×\cfrac{3}{4}\) = 45 minutes

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