From A a car travels in a straight line along a 10 km road and then immediately returns to A. The whole trip takes 20 minutes. Find the average speed of the car.

Step-by-step solution

1. 1

   Separate speed from velocity. Average speed uses the total path length travelled, while average velocity uses the displacement. The car returns to its starting point, so the displacement is zero — but the question asks for speed, so we must add both legs.

2. 2

   Add the two legs. $s=10+10=20\text{ km}$

3. 3

   Convert the time to hours. $t=20\text{ min}=\frac{20}{60}\text{ h}=\frac{1}{3}\text{ h}$

4. 4

   Divide total path by total time. $v_{tb}=\frac{s}{t}=\frac{20}{\tfrac13}=20\times 3=60\text{ km/h}$

5. 5

   Check the trap answer. Using only the one-way $10\text{ km}$ would give $30\text{ km/h}$ (option B), which is exactly the mistake of confusing displacement with distance. The correct choice is C, $60\text{ km/h}$.