Correct Answer - Option 2 : 12 p.m.
Given:
Three bells ring at regular intervals of 20 minutes, 30 minutes and 40 minutes respectively.
Calculations:
The time after which they will ring together = L.C.M of their ringing time
L.C.M (20, 30 and 40)
⇒ 120 minutes
⇒ 2 hr
They ring simultaneously at 10 a.m.
∴ They will ring simultaneously 2 hrs after 10 a.m i.e. at 12 p.m.