Pipe A can fill an empty cistern in 6 hours while along with Pipe B it can fill it up in 4 hours. Only Pipe B is on for an hour after which Pipe A is also turned on. How much time will it take in all to fill up the cistern?
1
4 hours 40 minutes
2
4 hours
3
4 hours 25 minutes
4
4 hours 15 minutes