Question

On gt road three consecutive traffic lights change after 3642 and 72 seconds if the lights are first switched on at 9


a.m. then at what time will they change simultaneously

Answer 1

The time they change simultaneously =LCMof 36,42,72
LCM=2*2*3*3*2*7=504
so they will meet after 504 seconds=8min 24 seconds 
the time they meet = 9:00:00 + 00:08:24=9:08:24 

for finding LCM,
prime factorise all the three numbers together