Question : A man goes from place A to B at a speed of 12 km/hr and returns from B to A at a speed of 18 km/hr. His average speed for the entire journey is:
Option 1: $14 \frac{2}{5} \mathrm{~km} / \mathrm{hr}$
Option 2: 15 km/hr
Option 3: 6 km/hr
Option 4: 30 km/hr
Correct Answer: $14 \frac{2}{5} \mathrm{~km} / \mathrm{hr}$
Solution :
Given: A man goes from place A to B at a speed of 12 km/hr and returns from B to A at a speed of 18 km/hr.
$\therefore$ Average speed = $\frac{\text{2 × (Speed to go to A to B) × (Speed to go to B to A)}}{\text{(Speed to go to A to B) + (Speed to go to B to A)}}$ = $\frac{2×12×18}{12+18}=14 \frac{2}{5} \mathrm{~km} / \mathrm{hr}$
Hence, the correct answer is $14 \frac{2}{5} \mathrm{~km}/\mathrm{hr}$.
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