Automobil

The car traveled three quarters of the total journey at a speed of 90 km/h and the remaining part of the journey at a speed of 50 km/h. Find its average speed.

Correct answer:

v =  75 km/h

Step-by-step explanation:

$$\begin{gathered} v_1=90\text{km/h} \\ {v}_2=50\text{km/h} \\ {s}_1=\frac{3}{4}s \\ {s}_2=\frac{1}{4}s \\ v=\frac{s}{t}=\frac{s}{s_1/v_1+s_2/v_2} \\ v=\frac{s}{t}=\frac{s}{\frac{3}{4}\cdot s/v_1+\frac{1}{4}\cdot s/v_2} \\ v=\frac{1}{\frac{3}{4}/v_1+\frac{1}{4}/v_2}=\frac{1}{\frac{3}{4}/90+\frac{1}{4}/50}=75\text{km/h} \end{gathered}$$

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