Problem 14 of 22num-014
Two lighthouse lamps flash together. After that, one flashes every seconds and the other every seconds. After how many seconds do they next flash together?
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Solution walkthrough1:21
- Each lamp flashes at the multiples of its own number. They flash together at a multiple of both and .
- First lamp: hop along the number line by . It flashes at , , , and so on.
- Second lamp: hop by . It flashes at , , and so on.
- The first number on both lists is . That is the least common multiple of and .
- Traps: divides both numbers, is a flash of only the first lamp, and is just .