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?  Choose the correct choice from the following options
A) 90 min  B) 100 min  C) 240 min  D) 120 min  E) 130 min

Rationale: 1 / 40 - ( 1 / 60 + 1 / 80 ) = - 1 / 240 third pipe can empty in 240 minutes answer : c
Answer: C