AP Police SI Exam Prelims 2018 Paper 1 (Mental Ability)

A pipe can fill a cistern in 12 minutes while a second pipe fills it in 15 minutes. But a third pipe can empty that completely filled cistern in 6 minutes. The fist two pipes are kept open for 5 minutes initially and then the third one is also opened. Then the further time (in minutes) taken to empty that cistern is