On a 600 m circular track, Riya runs at 5 m/s and Karan at 4 m/s. They start together from the starting point and run in the same direction. After how long will they first be together at the starting point again?

2 minutes 30 secondsCorrect answerYour answer4 minutes 30 secondsCorrect answerYour answer5 hoursCorrect answerYour answer10 minutesCorrect answerYour answer

Show the answer and explanation

CorrectNot quiteAnswer D

Correct answer: D

Lap times: Riya $600 \div 5 = 120$ s, Karan $600 \div 4 = 150$ s.
They are at the start together after $\operatorname{LCM}(120, 150) = 600$ s.
600 $s = 10$ minutes.

Worth noting

5 hours comes from multiplying the lap times ( $120 \times 150 = 18{,}000$ s); 4 min 30 s adds them.

Concept: LCM of lap times
Typical timeTypical: 75 s
Your timeYours: —

The rule this testsHCF and LCM word problems

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

On a 600 m circular track, Riya runs at 5 m/s and Karan at 4 m/s. They start together from the starting point and run in the same direction. After how long will they first be together at the starting point again?

Explanation