Four runners started running simultaneously from a point on a circular track. They took 280 seconds, 350 seconds, 420 seconds and 560 seconds, respectively, to complete one round. After how much time (in seconds) do they meet at the starting point for the first time?
Show answer & explanation
Correct answer: C
Explanation
The correct answer is option 3 i.e. 8400
280 = 23 × 5 × 7
350 = 2 × 52 × 7
420 = 22 × 3 × 5 × 7
560 = 24 × 5 × 7
LCM = 24 × 3 × 52 × 7
= 16 × 3 × 25 × 7
= 8400