Four bells ring at intervals of 5, 10, 12 and 15 minutes respectively. If they start ringing together at 3 o'clock, after what interval of time will they ring together and how many times will they ring together in 18 hours?
1
1 hour, 18 times
2
3 hours, 6 times
3
30 minutes, 36 times
4
More than one of the above
5
None of the above