AS Daily A Level Physics question
A student runs a 400 m lap in 80 s at constant pace. On the next lap they cover the same distance but their average speed is 25% higher. What is their new lap time?
Answer
The correct answer is D.
Correct: D — 64 s. With distance fixed, time is inversely proportional to average speed, so increasing speed by 25% (×1.25) means time divides by 1.25: 80 s ÷ 1.25 = 64 s. A treats the time as decreasing by 25% of 80 s (80 − 20 = 60 s), wrongly assuming a direct rather than inverse relationship. B comes from using a 20% increase (dividing by 1.2) or rough rounding, not the stated 25%. C multiplies the time by 1.25 (to 100 s), which assumes time increases with speed—the opposite of the correct inverse relationship. D correctly applies the inverse proportion: higher speed by ×1.25 gives time of 80/1.25 = 64 s.