Question

electric device makes a beep after every 15 minutes another device maximum after every 20 minutes there be together at 6 a.m. at what time will they must be together

Answer 1

Calculate LCM of 15 and 20 which comes out to be 60 hence they'll beep together again after 60 minutes or at 7:00 am.

Answer 2

Take lcm of 15 and 20 that is 60 min that means they will beep at 7 am