Three angles lie side by side along a straight line with no gaps between them. Two of them measure 40 degrees and 90 degrees. Enter the number of degrees in the third angle.

Explication

Angles arranged along a straight line make a half turn between them, so the three must reach 180 degrees; 40 plus 90 accounts for 130, and the remainder is 50. Entering 130 reports how much of the half turn has already been used rather than how much is still free.