alleryA man travels equal distances at 40 km per hour and at 60 km per hour. His average speed is:
A46 km per hour
B48 km per hour
C50 km per hour
D52 km per hour
Answer & Solution
Correct answer: B. 48 km per hour
1. Average speed is total distance divided by total time, never the mean of the two speeds.
2. Take each leg as 120 km, a convenient common multiple.
3. The first leg takes 120 / 40 = 3 hours and the second 120 / 60 = 2 hours.
4. So 240 km takes 5 hours.
5. The average speed is 240 / 5 = 48 km per hour. Answering 50 is the mean-of-speeds trap, and it is always too high.
_Source: syllabus Maths, Ch 11 'Direct and Inverse Proportions', S11.3 Inverse Proportion_
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