Two pipes A and B can separately fill a cistern in 45 minutes and 60 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?  Choices:
A) 90 min
B) 100 min
C) 72 min
D) 75 min
E) 130 min

Rationale: "1 / 40 - ( 1 / 45 + 1 / 60 ) = - 1 / 72 third pipe can empty in 72 minutes answer : c"
Answer: C