Average speed

The first third of the track was driven by a car at a speed of 15 km/h, the second third at a speed of 30 km/h, and the last third at a speed of 90 km/h. Find the average speed of the car.

Correct answer:

v =  27 km/h

Step-by-step explanation:

$$\begin{gathered} v_1=15\text{km/h} \\ {v}_2=30\text{km/h} \\ {v}_3=90\text{km/h} \\ v=\frac{s}{t}=\frac{s}{s/3/v_1+s/3/v_2+s/3/v_3} \\ v=\frac{s}{t}=\frac{3}{1/v_1+1/v_2+1/v_3} \\ v=\frac{3}{1/v_1+1/v_2+1/v_3}=\frac{3}{1/15+1/30+1/90}=27\text{km/h} \end{gathered}$$

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