Unit 2 · Number Theory
Three Flashing Lights — Count the Overlap
Lesson 16 of 18
About this lesson
Green+blue share a cycle every LCM(3,5)=15 min. Red joins every LCM(2,15)=30 min. Count the 15-min hits in the window, then subtract the 30-min hits.
Chapters
Question
A display has three lights that flash at different intervals.
The red light flashes every 2 minutes. The green light flashes every 3 minutes. The blue light flashes every 5 minutes.
The lights all flash together at 7:00 am.
How many times between 7:01 am and 9:59 am the same morning do the green and blue lights flash together without the red light also flashing?
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