Question

Tom and Amy set the alarms on their phones to sound at 6:45 am. Both alarms sound together at 6:45 am. Toms alarm sound every 9 minutes and Amy’s alarm sounds every 12 minutes. At what time will both alarms next sound together?

Answer 1

After 36 min , first alarm will ring 4 times (36=4*9) and the second alarm 3 times (36=3*12)
6.45- 6.54- 7.03- 7.12- 7.21   
6.45- 6.57- 7.09- 7.21

At 7.21 will ring together

Answer 2

Answer is 7:21am,
12x3=36
9x4=36
Hence 6:45am adding 36 mins equals to 7:21am