3. Two cyclists start training simultaneously. One starts from Skopje, and the other from Gevgelija, heading towards each other. When they are $180 \mathrm{~km}$ apart, a fly joins the training. It starts from the shoulder of one cyclist and flies to meet the other. It stops on the other's shoulder and immediately returns to the first cyclist. It repeats this until the two cyclists meet. The fly flies at a speed of $30 \mathrm{~km}$ per hour, while the cyclists move at a speed of $15 \mathrm{~km}$ per hour. How many kilometers will the fly fly during the training?
Solution. From the moment the fly joins the training to the moment the cyclists meet, $\frac{180 \mathrm{~km}}{2.15 \mathrm{~km} / \mathrm{h}}=6 \mathrm{~h}$ will pass. This means the fly will fly for $6 \mathrm{~h}$, or it will fly $6 \cdot 30 \mathrm{~km}=180 \mathrm{~km}$.