Given the task definition and input, reply with output. The task is to generate the correct answer for a given math problem.

Problem: two pipes a and b separately fill in 15 / 2 min and 5 min respactively and third pipe c can empty of 14 l / m if all the pipes are open when the cistern is full , it is emptied in 1 hr . how many litres does the cistern hold ?
40