Three angles meet at a single point and completely fill the space around it. Two of them measure 120 degrees and 150 degrees. What is the size of the third angle?

Explanation

Angles that fill the space around a point make one full turn of 360 degrees, so the third is 360 minus 120 minus 150, which comes to 90 degrees. The value 270 degrees merely adds the two stated angles together and describes the part of the turn already taken.