G7.3 A man travels $10 \mathrm{~km}$ at a speed of $4 \mathrm{~km} / \mathrm{h}$ and another $10 \mathrm{~km}$ at a speed of $6 \mathrm{~km} / \mathrm{h}$. If the average speed of the whole journey is $x \mathrm{~km} / \mathrm{h}$, find $x$.
$x=\frac{20}{\frac{10}{4}+\frac{10}{6}}=\frac{24}{5}$