Four alarms beep every 6, 8, 10 and 12 seconds. They beep together at the start. How many more times will they beep together in the next 30 minutes, up to and including the 30-minute mark, NOT counting the beep at the start?

14Correct answerYour answer15Correct answerYour answer16Correct answerYour answer30Correct answerYour answer

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CorrectNot quiteAnswer B

Correct answer: B

They beep together every $\operatorname{LCM}(6, 8, 10, 12) = 120$ $s = 2$ minutes.
In 30 minutes: $30 \div 2 = 15$ joint beeps after the start (the last one exactly at 30 minutes).

Worth noting

16 also counts the beep at the start, which the question excludes; 14 leaves out the joint beep at exactly 30 minutes, which the question includes.

Ten questions from HCF & LCM, at your own pace.

Explanation