There is a string of red and blue lights strung around a window. the red lights blink every 3 seconds and the blue lights blink every 5 seconds. how many times were they blink at the same time in one minute?

The red lights will blink at seconds 3,6,9,12, etc.(all factors of 3 up to 60)
The blue lights will blink at seconds 5, 10, 15, etc.(all factors of 5 up to 60)

Considering this, they will blink simultaniously at intervals only common to factors of 5 and 3 ie. 15, 30, 45, and 60 only.

4 times in 1 minute

Hope this helps:)

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