Three situations, mixed on one page: the third angle of a triangle when two are known, the second of two angles on a straight line, and the third of three angles meeting at a point. They rest on three facts — a triangle’s angles add to 180°, a straight line is 180°, and a full turn is 360°.
Every question is assembled from its parts rather than drawn and then checked, so the angles always add up exactly and no missing angle is ever 0°.
Tear the three corners off a paper triangle and lay them side by side: they form a straight line. That is the fact these questions use, and it holds for every triangle.
Add the two you were given and subtract from 180. If two angles are 85° and 10°, the third is 180 − 95 = 85°.
Angles on a straight line add to 180°, half a turn. Angles around a point add to 360°, a full turn. The sheet mixes both so a student has to read which one is being asked.
Fourth grade for adding and subtracting angles (4.MD.C.7); eighth grade for the triangle angle sum as a rule (8.G.A.5). Setting the angle step to 10° keeps the arithmetic easy while the idea is new.