Question

Three lights go on together first changes every 5 sec second every 6sec third every 8 sec when is the next time they go on together

Answer 1

In order to calculate, you need to find their LCM
LCM of 5,6 and 8 is equal to 120

So, That time will be 120 sec = 2 min.
In short, Your Final Answer would be 2 min.

Hope this helps!