A lighthouse blinks regularly 5 times a minute. A neighboring lighthouse blinks regularly 4 times a minute. If they blink simultaneously, after how many seconds will they blink together again?

A. 20
B. 24
C. 30
D. 60
E. 300

Question

A lighthouse blinks regularly 5 times a minute. A neighboring lighthouse blinks regularly 4 times a minute. If they blink simultaneously, after how many seconds will they blink together again?

 

A.       20
B.       24
C.       30
D.      60
E.       300

anonymous · Answer

So first off, if the first lighthouse blinks 5 times per minute, then how many seconds are there between blinks?

anonymous · Answer

Yes, the blinks would happen every 12 seconds.

anonymous · Answer

And if the second lighthouse blinks 4 times per minute, then how many seconds between blinks are there for the second lighthouse?

anonymous · Answer

Well, there are 60 seconds in a minute, and so if the lighthouse blinks 4 times in every 60 seconds, then it blinks once every 60/4 seconds, or once every 15 seconds.

anonymous · Answer

And so the two lighthouses will blink at the same time after an amount of time which is the least common multiple of 12 and 15. What is the least common multiple of 12 and 15.

anonymous · Answer

?

anonymous · Answer

60 is the LCM of 12 and 15. So that would be our answer!!

anonymous · Answer

thanks alot @ⒶArchie☁✪

anonymous · Answer

No worries! :)