Shouldn't the answer here be 20, not 30?

Greg Mat+ Questions/Issues
1

This is from PrepSwift’s “Three Ringing Bells” quiz.

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2

Indeed, an odd one. That’s fixed.

3

Hi, How were you able to solve this? I found the total LCM and then choose 2 bells with their LCM and subtracted that. But I didn’t get the right answer.

4

You should not be finding the total LCM. Hint: what is the LCM of any of these two balls?

5

Thanks. What would I do after solving any of them? How do I find out the seconds that elapsed?

6

Suppose we only had two bells. What would you do? Find the LCM.
In this case, we have three. Take each pair and find the LCM. The smallest amongst the three distinct pairs is your answer.