Question

A car travels along a straight line. For the first half it travels $20km/h$ and for second half it travels $50km/h$. What is the mean speed of that car? (in km/h)

• A. $29.36$
  
• B. $24.36$
  
• C. $28.6$
  
• D. $25.2$
  

Answer 1

The correct option is C $28.6$

The average speed is defined as the ratio of distance traveled and time traveled.

$v_{av}=\frac{d}{t}\to 1$

Let us assume that the car travels distance d.

Then we know that the first half, which means the distance $d/2$, the car travels with the speed of $20㎞/h$

$\therefore t_1=\frac{d/2}{20}=\frac{d}{40}$

Then the car travels the second half-distance $d/2$ with speed of $50㎞/h$

$\therefore t_2=\frac{d/2}{50}=\frac{d}{100}$

Total time the car travels is the sum of $t_1$ and $t_2$

$t=t_1+t_2=\frac{d}{40}+\frac{d}{100}$

Substitute this in equation $1$

$v_{av}=\frac{d}{\frac{d}{40}+\frac{d}{100}}=\frac{1}{\frac{1}{40}+\frac{1}{100}}$

$=\frac{200}{7}=28.6㎞/h$