A triangle's angles will always equal up to 180. Since you already have to of the angles, you will add them up together. 30+20=50. Then, you take 180 and subtract it by the sum of the two previous angles. 180-50=130. Now you know the last angle is 130. You also know that the missing angle is also 130. The reason for that is because the two angles are the exact same if you look at it.
