The capacity of a cistern is _________ litres. Two inlet pipes can fill it separately in 24 minutes and 40 minutes respectively. An outlet pipe is opened which can drain the cistern at 30 litres/minute. If all the pipes are opened together then the cistern fills in one hour.
1
300 litres
2
600 litres
3
400 litres
4
500 litres
5
None of these