180

here's a fun visualization: http://www.mathsisfun.com/proof180deg.html

Here is a way to remember that the angles of a triangle sum to 180 degrees.

Imagine a squat triangle, with two very acute angles and one very obtuse angle.  If this triangle is very stout, then the acute angles are ~0 and the obtuse angle is ~180 degrees.

The sum of the measures of the angles in a triangle is 180