What's the average speed of the second car?

What's the average speed of the second car?

Question

The winning car in a Grand Prix race covers the $360\,\text{km}$ course at an average speed of $150\,\text{km/h}$. The second car takes $6$ minutes longer than the first. Find the average speed of the second car.

Solution

As the first car has an average speed of $150\,\text{km/h}$, it needs $\frac{360\,\text{km}}{150\,\text{km/h}} = 2.4\,\text{h}$ for the whole course.

Now the second car takes $6\,\text{min} = 0.1\,\text{h}$ longer than the first car. Hence it needs $2.5\,\text{h}$ for the full course. This makes for an average speed of $\frac{360\,\text{km}}{2.5\,\text{h}} = 144\,\text{km/h}.$