Zeljko travelled at $30 \mathrm{~km} / \mathrm{h}$ for 20 minutes and then travelled at $20 \mathrm{~km} / \mathrm{h}$ for 30 minutes. How far did he travel, in kilometres?
Since 20 minutes is $\frac{1}{3}$ of an hour and Zeljko travelled at $30 \mathrm{~km} / \mathrm{h}$ for $\frac{1}{3}$ of an hour, then Zeljko travelled $30 \cdot \frac{1}{3}=10 \mathrm{~km}$ during this portion of the trip.

Since 30 minutes is $\frac{1}{2}$ of an hour and Zeljko travelled at $20 \mathrm{~km} / \mathrm{h}$ for $\frac{1}{2}$ of an hour, then Zeljko travelled $20 \cdot \frac{1}{2}=10 \mathrm{~km}$ during this portion of the trip.

In total, Zeljko travelled $10+10=20 \mathrm{~km}$.

ANSWER: 20