What is the angle sum of a quadrilateral?

90 x 4, or 360. You can see this in a square, which has 4 90 degree angles. Any time you increase or decrease the length of any of the 4 sides, one angle is increased and another decreases by the same amount. So the total of all four angles will always be 360 degrees.

Interestingly, this also occurs in triangles, but the total for a triangle is 180 degrees.