We have $C D=80-50=30$ and $A B=80-45=35$. Therefore, $B C=80-35-30=15$.

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2 -

![](https://cdn.mathpix.com/cropped/2024_05_01_ed653f4a7a838e105fd2g-03.jpg?height=279&width=577&top_left_y=1848&top_left_x=180)

From the statement, we have: \( AC = 50 \), \( BD = 45 \), and \( AD = 80 \). From the figure, it follows that \( BC = AC - AB \), so \( BC = 50 - AB \).

Therefore, it is enough to calculate \( AB \). Note from the figure that \( AB = AD - BD \), and thus, \( AB = 80 - 45 = 35 \).

Finally, \( BC = 50 - 35 = 15 \text{ km} \).

Solution 3 - From the figure, we have \( 45 - BC = 80 - 50 \). Therefore, \( BC = 15 \text{ km} \).