The sum of interior angles in a triangle is how many degrees?

• A. 360
• B. 180
• C. 90
• D. 270

Answer: (B)

In a flat plane, the interior angles of any triangle add up to 180 degrees. A clear way to see this is to draw a line parallel to the base through the top vertex; the other two interior angles become corresponding angles with the two base angles, so all three angles lie on a straight line and sum to 180 degrees. The other numbers don’t fit because 360 is a full turn, 90 is just a single right angle, and 270 isn’t the sum of a triangle’s interior angles. (Note: on curved surfaces, the sum can differ, but in Euclidean geometry it is 180 degrees.)