Avg speed 2

A car covers four successive 3 km stretches at 10 kmph, 20 kmph, 30 kmph, and 60 kmph, respectively. What is the average speed over this distance?

Correct answer:

v =  20 km/h

Step-by-step explanation:

$$\begin{gathered} s=3\text{km} \\ {v}_1=10\text{km/h} \\ {v}_2=20\text{km/h} \\ {v}_3=30\text{km/h} \\ {v}_4=60\text{km/h} \\ {t}_1=\frac{s}{v_1}=\frac{3}{10}=0.3\text{h} \\ {t}_2=\frac{s}{v_2}=\frac{3}{20}=0.15\text{h} \\ {t}_3=\frac{s}{v_3}=\frac{3}{30}=\frac{1}{10}=0.1\text{h} \\ {t}_4=\frac{s}{v_4}=\frac{3}{60}=\frac{1}{20}=0.05\text{h} \\ t=t_1+t_2+t_3+t_4=0.3+0.15+0.1+0.05=\frac{3}{5}=0.6\text{h} \\ {s}_4=4\cdot s=4\cdot 3=12\text{km} \\ v=\frac{s_4}{t}=\frac{12}{0.6}=20\text{km/h} \end{gathered}$$

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