1. **State the problem:** Find the value of the missing interior angle $x$ in a triangle with angles 55°, 25°, and $x$°. 2. **Formula and rule:** The sum of interior angles in any triangle is always 180°. So, $$55 + 25 + x = 180$$ 3. **Calculate $x$:** $$55 + 25 = 80$$ $$80 + x = 180$$ 4. **Isolate $x$:** $$x = 180 - 80$$ 5. **Final answer:** $$x = 100$$ So, the missing angle $x$ is 100°.