A motor vehicle travelled the first third of a distance s at a speed of v₁ = 10 kmph, the second third at a speed of v₂ = 20 kmph and the last third at a speed of v₃ = 60 kmph.
Determine the mean speed of the vehicle over the entire distance s.

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10 kmph

15 kmph

18 kmph

12 kmph

answer is D.

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Detailed Solution

$\begin{array}{l} v_{avg} = \frac{3 v_{1} v_{2} v_{3}}{v_{1} v_{2} + v_{2} v_{3} + v_{3} v_{1}} \\ v_{avg} = \frac{3 \times 10 \times 20 \times 60}{10 \times 20 + 20 \times 60 + 60 \times 10} \end{array}$
v_avg = 18kmph

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