Two similar triangles are given. The length of one side of the smaller triangle is 10 km, and the corresponding side of the larger triangle is 60 km. Another side of the smaller triangle is 20 km. We need to find the length $u$ of the corresponding side of the larger triangle.

1. Problem Description

Two similar triangles are given. The length of one side of the smaller triangle is 10 km, and the corresponding side of the larger triangle is 60 km. Another side of the smaller triangle is 20 km. We need to find the length $u$ of the corresponding side of the larger triangle.

2. Solution Steps

Since the triangles are similar, the ratio of corresponding sides is constant. We can write the proportion:

$\frac{10}{60} = \frac{20}{u}$

Cross-multiply to solve for $u$:

$10u = 60 \times 20$

$10u = 1200$

$u = \frac{1200}{10}$

$u = 120$

3. Final Answer

The missing length $u$ is 120 kilometers.