Answer :

Solution :

Given : Three bells P, Q and R rings periodically in a school.
P rings in every 20 minutes, Q rings in every 30 minutes and R rings in every 50 minutes.
Bells, P, Q and R will ring after the respective Least Common Multiple of ringing time intervals of them.
Least Common Multiple of 20 min, 30 min and 50 min is 300 min.
Where 300 minutes is equals to 5 hours.
As, all the three bells rangs at 12:00 PM then after 12 PM, the three bells ring together again, after 5 hours at 5 PM.
Hence, the correct option is (B).