Question: A pipe P can fill a tank in 12 min and another pipe R can fill it in 15 min. But, the 3rd pipe M can empty it in 6 min. The 1st two pipes P and R are kept open for double the 2.5 min in the beginning and then the 3rd pipe is also opened. In what time is the tank emptied?
Options:
A) 30 min
B) 25 min
C) 45 min
D) 35 min
E) None of these
Show Answer
Answer:
Correct Answer: C
Solution:
- According to the question,
Double the $ 2.5\min =5\min $
Now, part filled in 5 min $ =5\times ( \frac{1}{12}+\frac{1}{15} ) $
$ =5\times ( \frac{5+4}{60} )=5\times \frac{9}{60}=\frac{3}{4} $
Part emptied in 1 min when P, R and M, all are opened.
$ =\frac{1}{6}-( \frac{1}{12}+\frac{1}{15} )=\frac{1}{6}-( \frac{5+4}{60} )=( \frac{1}{6}-\frac{3}{20} )=\frac{1}{60} $
One-sixtieth part is emptied in 1 min.
$ \therefore $ Three-fourth part will be emptied in
$ 60\times \frac{3}{4}=15\times 3=45\min $