If a triangle has angles measuring 30° and 60°, what is the measure of the third angle?

In any triangle, the sum of all three interior angles must equal 180 degrees. Given that two angles of the triangle measure 30 degrees and 60 degrees, we can find the measure of the third angle by using this fundamental property of triangles.

First, we add the measures of the two known angles:

30° + 60° = 90°

Next, we subtract this sum from the total angle measure for a triangle:

180° - 90° = 90°

Thus, the measure of the third angle is 90 degrees. This makes sense not only mathematically but also in the context of triangles, as a triangle can have one right angle, making it a right triangle. The correct answer, therefore, is 90°.