The outer corners of the triangle are 100 and 120 * Find the inner corner at the third vertex.

The sum of the outer and inner angles at one vertex is 180 °.
Based on this property, we find the internal angles at the external angles known by the condition:
∠1 = 180 ° – 100 ° = 80 °;
∠ 2 = 180 ° – 120 ° = 60 °.
The two angles of the triangle are known, we find the third inner corner:
∠ 3 = 180 ° – (∠ 1 + ∠ 2) = 40 °
Answer: The degree measure of the third internal angle is 40 °.