Detailed Instructions: Given a math problem with context and a question and 5 answer choices, the task is to provide the correct answer choice based on the problem. You must choose one of the given answer choices by letter: a, b, c, d, or e; anything else is invalid.
Problem:Problem: 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 ?
Options: a. 90 min, b. 100 min, c. 240 min, d. 120 min, e. 130 min
Solution:
c