Question

A pipe can fill a tank in 12 minutes and another pipe in 15 minutes but a third pipe 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

Answer 1

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YOUR ANSWER IS



Part of the tank filled by first pipe in 1 min = 112112
Part filled by second pipe in 1 min = 115115
Part emptied by third pipe in 1 min = 1616

Assuming that initially the cistern was empty and then first two pipes were kept open for 5 min

Part filled by first two pipes together in 1 min = 112+115=320112+115=320
Part filled by first two pipes together in 5 min = 5×320=345×320=34

Part emptied when all the three pipes are open, in 1 min 
= 16−(112+115)=16−320=16016−(112+115)=16−320=160

Therefore, time required to empty the tank after all the three pipes are open
=(34)(160)=34×60=45=(34)(160)=34×60=45