Answer:3:39 PM
Explanation:
Given
Cell phone Alarm is set to ring every 30 minutes
Computer alarm is set to ring every 20 minutes
Clock alarm is set to ring every 45 minutes
All alarms ring at 12:39 so next time they ring simultaneously is given by
LCM of (30, 20, 45)=180
Thus all alarms will ring simultaneously after 180 min i.e. at 3:39 PM