A circular cycling track has a circumference of 1km. If two cyclists starting simultaneously have constant speeds of 15km/hr and 20km/hr, how long will it take them to meet at the starting point again?

Explanation

The key to solving this problem is understanding the concept of relative speed in circular motion. The relative speed of the two cyclists is the difference in their speeds, which is 5 km/hr. When we convert this to km/minute, we get 0.0833 km/minute. Since the circumference of the track is 1 km, we can use the formula time = distance / speed to calculate that it will take them 1/0.0833 = 12 minutes to meet again at the starting point. The other options represent the time it would take if they were moving at a combined speed of 10 km/hr (Option A), approximately 6.67 km/hr (Option B), and 5 km/hr (Option C), none of which are the case.