Two lighthouses flash their lights every 20 and 30 seconds respectively. given that they flash together at 8 p. m., at what time will they next flash together?

Do you remember when you were learning about
Least Common Multiple (LCM) ?  And you were
wondering what on Earth you would ever need it for ?

THIS IS IT !

The amount of time after 8 PM when the lights will flash
together again is the LCM of 20 seconds and 30 seconds.
Do you remember how to find it ?

... When 20 is prime-factored, you have  2 · 2 · 5 .

... When 30 is prime-factored, you have  2 · 3 · 5 .

... So the Least Common Multiple of 20 and 30 is  2 · 2 · 3 · 5  =  60 .

The lighthouses will flash together again in 60 seconds after 8 PM.
That'll be  8:01 PM.  

)because it's going in order of the math sign