Two pipes A and B can fill a cistern in 20 min. and 30 min. respectively, and a third pipe C can empty it in 36 min. How long will it take to fill the cistern if all the three are opened at the same time ?
Solution: Formula: If a pipe can fill a tank in x hours and another pipe can empty the full tank in y hours, then on opening both the pipes, the net part filled in 1 hour = (1/x - 1/y) (where y > x).
Then, part filled in 1 min. = (1/20 + 1/30) - 1/36
= 5/60 -1/36
= 1/12 - 1/36
.'. Cistern will be filled in 18 min.
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