Three of the four interior angles of a quadrilateral measure 90 degrees, 100 degrees and 80 degrees. What is the fourth angle?

Explanation

A quadrilateral splits into two triangles, so its interior angles total 360 degrees and the fourth is 360 minus 90 minus 100 minus 80, leaving 90 degrees. Choosing 270 degrees states how much of the total the three stated angles have already claimed instead of what is left for the fourth.