IBPS PO Mains 2017 Paper for online practice

A bath can be filled with the cold water pipe in 10 minutes and by hot water pipe in 15 minutes (independently each). 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 opened he now closes it. In 4 minutes more, the bath is full. In what time would be the waste pipe empty it?