two lighthouses flash their lights every 20 seconds and 30 s respectively. given that they flashed together at 4.10 pm, when will they next flash together?
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First one flashes at 8:00:20, 8:00:40, 8:01:00
2nd one flashes at 8:00:30, 8:01
So they flash together at 8:01
This is because the LCM of 20 and 30 is 60, so they sync up again after 60 seconds
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