Two runners on a circular track: time to first meet at the starting point

The same question, asked 3 times: 14 Jul 2023 S1*, 14 Jul 2023 S2*, 17 Sep 2024 S3. Every wording below, oldest first.

Try each one first, then open its answer.

Question 1
14 Jul 2023 S1
74 $Two runners, Sony and Mony, start running on a circular track of length 200 m at speeds of 18 and 24 km/h, respectively, in the same direction. After how much time from the start will they meet again at the starting point?$
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Question 2
14 Jul 2023 S2
51 $In a race around a circular cycling track of 75 km, two cyclists are riding at a speed of 30 km/h and 25 km/h. After what time (in hours) will they meet at the point from where they started their journey?$
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Question 3
17 Sep 2024 S3
55 $Arvind and Chetan started running simultaneously from the same point in the same direction on a circular track of length 210 m. If Arvind takes 180 seconds to complete one round, and Chetan takes 420 seconds to complete one round, after how much time will they meet for the first time at the starting point on the track?$
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Question 1