Multiple choice

Sarah is a professional cyclist who is training for an upcoming marathon. She practises on a circular track and completes one lap at an average speed of 36 km/hr, taking 10 minutes per lap. Sarah wants to improve her performance and aims to complete each lap in 8 minutes to match the pace required for her upcoming race. By how much should she increase her speed (in km/hr) to achieve this new lap time?

  1. 7.5 km/hr

  2. 8 km/hr

  3. 9 km/hr

  4. 10 km/hr
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Current speed = 36 km/hr. Time = 10 min. Distance per lap = 36 * (10/60) = 6 km. Required speed for 8 min = 6 km / (8/60) hr = 6 * (60/8) = 360/8 = 45 km/hr. Increase = 45 - 36 = 9 km/hr.

AI explanation

First, find the track length using the formula Distance equals Speed multiplied by Time, where the speed is 36 km/hr and the time is 10 minutes (or 1/6 hours), giving a distance of 36 / 6 equals 6 km. To complete this 6 km lap in 8 minutes (which is 8/60 or 2/15 hours), her new required speed is Distance divided by Time, giving 6 / (2/15) equals 45 km/hr. The required increase in her speed is the difference between the new speed and the old speed, which is 45 - 36 equals 9 km/hr.

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