The three interior angles of any triangle add to 180 degrees. If two angles are 50 and 70, the third is 60. Angle facts as a set are in lines and angles.
Use 180, then stop
Subtract the two given angles from 180. If the figure is a right triangle, one angle is already 90, so the other two add to 90. An equilateral triangle is three 60s, in isosceles and equilateral triangles.
Complementary and supplementary pairs are neighbors, in complementary and supplementary angles. The short list is in ACT Math formulas.
The bottom line
Three interior angles: 180. Right triangle: the other two add to 90.
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