A bath tub can be filled by the cold water and hot water pipes in 10 mins and 15 mins, respectively. A person leaves the bathroom after turning on both pipes simultaneously and returns at the moment when the bath should be full. Finding, however, that the waste pipe has been open, he then closes it. In exactly 4 mins more the bath is fill. In how much time would the waste pipe empty the full bath, if it alone is opened.
The answer given to the above question is 15 mins and not 9 mins. How?
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