Posted by Kenny on Sunday, February 3, 2008 at 7:46pm.
Three lighthouses on three nearby islands flash their lights every evening. On one island, the light flashes every 45 seconds. The lighthouse on the second island flashes every 2 minutes. The third lighthouse flahses a light every 1 and 1/4 minutes. Suppose the lights flash together at 8:35pm. At what time will they flahs together agian? Explain your answer.

Math  Damon, Sunday, February 3, 2008 at 9:09pm
First the times in seconds.
Then least common multiple
45 seconds, 120 seconds, 75 seconds
factor them all
3 *3 * 5
2 * 2 * 2 * 3 * 5
3 * 5 * 5
so we need
2 * 2 * 2 * 3 * 5 * 5
thet is
8*3*25
=600 seconds
= 10 minutes 
Math  Anonymous, Thursday, May 24, 2012 at 10:26pm
8:45

Math  Daniel, Wednesday, September 21, 2016 at 7:29pm
It's actually 2*2*2*3*3*5*5 = 1800 seconds which is 30 minutes. It will be 9:05 when they all flash together again.