Question 103708
Three lighthouses on three nearby islands flash their lights every evening. On one island, the light flashes every 45 seconds. The second lighthouse flashes every 2 minutes. the third flashes every 1 1/4 minutes. Suppose all three lights flash together at 8:35 pm. At what time will all three lights flash together again?
:
put all the flash times in seconds
45, 120, 75
:
Find the LCM (lowest common multiple) of the 3 numbers:
Prime factors will help us do this:
45: 3,3,5
120: 2,2,2,3,5
75: 3,5,5
:
LCM: 2*2*2*3*3*5*5 = 1800 secs
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We know 1 hr = 3600 sec, therefore 1800 sec = 1/2 hr
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They will flash simultaneously at 8:35 + :30 = 9:05
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