If the measure of angle 1 is 40, the measure of angle 3 is 100.
You can find this by first assuming that since the measure of angle 1 is 40, the measure of angle 2 is also 40. This is because they have marked congruence.
You also know that the sum of angles 1, 2 and 3 should be 180. That's because they create a straight line when put together. Therefore, you can solve using the following equation.
Angle 1 + Angle 2 + Angle 3 = 180
40 + 40 + Angle 3 = 180
80 + Angle 3 = 180
Angle 3 = 100
