two pipes a and b can separately fill a cistern in 60 minutes and 80 minutes respectively . there is a third pipe in the bottom of the cistern to empty it . if all the three pipes are simultaneously opened , then the cistern is full in 40 minutes . in how much time , the third pipe alone can empty the cistern ?
1 / 40 - ( 1 / 60 + 1 / 80 ) = - 1 / 240 third pipe can empty in 240 minutes answer : c
inverse(subtract(add(inverse(60), inverse(80)), inverse(40)))