1. **Stating the problem:** We have a right-angled triangle with one angle marked as 90° and two other angles labelled as $f$ and $o$. We want to understand the relationship between these angles. 2. **Labelling:** We introduce variables $f$ and $o$ to represent the two unknown angles in the triangle. This is called labelling. 3. **Formula used:** The sum of angles in any triangle is always 180°. Therefore, we use the formula: $$f + o + 90^\circ = 180^\circ$$ 4. **Solving for $f + o$:** $$f + o = 180^\circ - 90^\circ$$ $$f + o = 90^\circ$$ 5. **Explanation:** Since one angle is 90°, the other two angles $f$ and $o$ must add up to 90°. This is a fundamental property of right-angled triangles. **Final answer:** $$f + o = 90^\circ$$