Given:
Measure of angle B = 120 degrees
Measure of angle C = 40 degrees
Let's find the measure of the third angle of the triangle, angle A.
To find the measure of the third angle, apply the Triangle Angle Sum theorem which states that the sum of all interior angles of a triangle is 180 degrees.
Thus, we have:
Measure of angle A + Measure of angle B + Measure of angle C = 180°
Measure of angle A + 120° + 40° = 180°
Measure of angle A + 160°= 180°
Measure of angle A = 180° - 160°
Measure of angle A = 20°
Therefore, the measure of the third angle is 20 degrees.
ANSWER:
20 degrees.