A completes a lap of a circular track in 60 seconds and B in 84 seconds. They start together from the starting point. After how many seconds are they next at the starting point together?

144 secondsCorrect answerYour answer210 secondsCorrect answerYour answer420 secondsCorrect answerYour answer5,040 secondsCorrect answerYour answer

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CorrectNot quiteAnswer C

Correct answer: C

A is at the start every 60 seconds and B every 84 seconds, so they are there together at a common multiple.
LCM $(60, 84) = 420$ seconds, which is 7 minutes.

Worth noting

5,040 multiplies 60 by 84; it is a common multiple, but not the least.

Ten questions from Races & Games, at your own pace.

Explanation