Question

Stoplight A changes color every 45 seconds and Stoplight B changes color every 80 seconds. Both stoplights changed color 60 seconds ago. After how many seconds will the stoplights change color at the same time again.​

Answer 1

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Answer:

660 seconds

Explanation:

The least common multiple of 45 seconds and 80 seconds is ...

45×80/GCF(45,80) = 45×80/5 = 720

The next change will be 720 seconds after the last one, so 720 -60 = 660 seconds from now.

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Additional comment

You can find the least common multiple by looking at the factors of 45 and 80:

• 45 = 5×9
• 80 = 5×16

The LCM will include one copy of the common factor and all of the unique factors:

LCM(45, 80) = 5×9×16 = 720

You can see that the greatest common factor is 5, and that the LCM is the product (5×9)(5×16) divided by 5:

LCM(a, b) = a×b/GCF(a, b)