The Question
One lighthouse flashes every 12 seconds, another every 18 seconds. They flash together at 8:00:00. When do they next flash together, and how many times do they flash together in the next 3 minutes?
The Answer
Reveal the Answer
LCM(12, 18) = 36 seconds. Three minutes is 180 seconds, and 180 ÷ 36 = 5. Worth asking whether the flash at 8:00:00 itself counts — a good exam-technique discussion.
Now watch the solution
Practice Sheet + Marked Answers
The four-question practice sheet, the solutions in our teaching voice and a self-marking quiz for this question are part of The Question Desk course — every school day of the term, with progress you can see. Enrolled group students get it included.
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