Posted by chelsea on Sunday, September 26, 2010 at 4:38am.
a light house flashes every 56 seconds. Another lighthouse flashes every 40 seconds. At 9 p.m. they both flash t the same time. What time will it e when they next both flash at the same time?

maths  Reiny, Sunday, September 26, 2010 at 7:04am
first lighthouse will flash in 56 sec, then 112, 168 , etc seconds
second lighthouse will flash in 49 sec, then 80, 120, etc seconds
so this is just like finding the LCD of 56 and 40
56 = 2x2x2x7
40 = 2x2x2x5
LCD = 2x2x2x5x7 = 280
so they will flash together 280 seconds from now or 4 minutes and 40 seconds from 9 pm
which would be 9:04:40 pm
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