On a piece of paper, use a protractor to construct a triangle with angle measures of 40° and 60º.

what is the measure of the third angle?

60°
80°
100°
180°

Considering that the sum of the measures of the internal angles of a triangle is of 180º, the third angle measures 80º.

A triangle is composed by three internal angles, and the sum of their measures is of 180º.

In this problem:

Since the sum of the measures is of 180º:

The third angle measures 80º.

A similar problem is given at link