We recall that time $=\frac{\text { distance }}{\text { speed }}$. Travelling $x \mathrm{~km}$ at $90 \mathrm{~km} / \mathrm{h}$ takes $\frac{x}{90}$ hours. Travelling $x \mathrm{~km}$ at $120 \mathrm{~km} / \mathrm{h}$ takes $\frac{x}{120}$ hours. We are told that the difference between these lengths of time is 16 minutes. Since there are 60 minutes in an hour, then 16 minutes is equivalent to $\frac{16}{60}$ hours. Since the time at $120 \mathrm{~km} / \mathrm{h}$ is 16 minutes less than the time at $90 \mathrm{~km} / \mathrm{h}$, then $\frac{x}{90}-\frac{x}{120}=\frac{16}{60}$. Combining the fractions on the left side using a common denominator of $360=4 \times 90=3 \times 120$, we obtain $\frac{x}{90}-\frac{x}{120}=\frac{4 x}{360}-\frac{3 x}{360}=\frac{x}{360}$. Thus, $\frac{x}{360}=\frac{16}{60}$. Since $360=6 \times 60$, then $\frac{16}{60}=\frac{16 \times 6}{360}=\frac{96}{360}$. Thus, $\frac{x}{360}=\frac{96}{360}$ which means that $x=96$.