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?

Answer 1

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