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.

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