Asked by Matt

I am very confused on how to figure out the below math problem:

A traffic light changes every 30 seconds. Another traffic light changes every 40 seconds. Both lights just changed. After how many minutes will both lights change at the same time again?

Please help me.
Answered by Ms. Sue
Find the least common multiple of 30 and 40.

30, 60, 90, 120, 150
40, 80, 120, 160




Answered by Matt
The least common multiple of 30 and 40 is 120.

How did you know that I need to find the least common multiple of 30 and 40.

Would 120 be the answer? So both lights would change at the same time again after 120 minutes.
Answered by Ms. Sue
You need to know what time they'd change at the same time.

Yes, 120 is correct.
Answered by Matt
What was the indicator in the word problem is what I am trying to figure out that told you multiple?
Answered by Ms. Sue
I just thought about it. No magic tricks.
Answered by Matt
The reason why I was asking is I printed out a paper to look out for key words in math word problems.

Thank you for helping me solve the problem.