Question

At 10 a.m., Ali and John set their alarm clocks to ring. Ali set his alarm
clock to ring every 15 minutes. John set his alarm clock to ring every 20 minutes. At what time will it take for both their alarm clocks to ring together for the first time?

Answer 1

15 = 3*5
20 = 4*5
so multiply 15 by 4 and multiply 20 by 3

Answer 2

or ...
Ali's will ring at the following times:
15, 30, 45, 60, 75, ....
John's will ring at
20, 40, 60, ...

so they will both ring 60 minutes from 10:00, that is
they will both ring at 11:00 am