Average speed

The car drove on one section of the highway for half an hour at a speed of 80 km/h. Then he reduced his speed to 60km/h. It went three-quarters of an hour at a speed of 60 km/h. What was the average speed of the car?

Correct answer:

v =  68 km/h

Step-by-step explanation:

$$\begin{gathered} v_1=80\text{km/h} \\ {t}_1=1/2=\frac{1}{2}=0.5\text{h} \\ {v}_2=60\text{km/h} \\ {t}_2=3/4=\frac{3}{4}=0.75\text{h} \\ {s}_1=v_1\cdot t_1=80\cdot \frac{1}{2}=\frac{80\cdot 1}{2}=\frac{80}{2}=40\text{km} \\ {s}_2=v_2\cdot t_2=60\cdot \frac{3}{4}=\frac{60\cdot 3}{4}=\frac{180}{4}=45\text{km} \\ v=\frac{s_1+s_2}{t_1+t_2}=\frac{40+45}{\frac{1}{2}+\frac{3}{4}}=68\text{km/h} \end{gathered}$$

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