Two ladders, each 5 m long, lean against two different vertical walls…

Step 1. The triangles. In each triangle the wall and the ground meet at a right angle. The ladder is the hypotenuse.

Step 2. The equal parts.

1. The right angles are equal.
2. The hypotenuses are equal: both ladders are $5\ \text{m}$ long.
3. The heights on the walls are equal: both are $4\ \text{m}$.

Step 3. The rule. The two triangles are congruent by RHS. By CPCT the distances along the ground are equal.

Step 4. The number. The second foot is also $3\ \text{m}$ from its wall.