1. Given a triangle with angles 40°, 100°, and $x$ degrees, we need to find the value of $x$. 2. Recall the rule that the sum of the interior angles of any triangle is always 180°. 3. Write the equation for the sum of angles: $$40 + 100 + x = 180$$ 4. Simplify the left side: $$140 + x = 180$$ 5. Subtract 140 from both sides to isolate $x$: $$x = 180 - 140$$ 6. Calculate the value: $$x = 40$$ 7. Therefore, the value of $x$ is 40.