I3.4 A car has already travelled $40 \%$ of its journey at an average speed of $40 c \mathrm{~km} / \mathrm{h}$. In order to make the average speed of the whole journey become $100 \mathrm{~km} / \mathrm{h}$, the speed of the car is adjusted to $d \mathrm{~km} / \mathrm{h}$ to complete the rest of the journey. Find $d$.
Let the total distance be $s$.
$$
\begin{array}{l}
\frac{s}{\frac{0.4 s}{40(2)}+\frac{0.6 s}{d}}=100 \\
\Rightarrow \frac{1}{200}+\frac{3}{5 d}=\frac{1}{100} \\
\Rightarrow \frac{120}{200 d}=\frac{1}{200} \\
\Rightarrow d=120
\end{array}
$$