In the figure a square, a circle and an ellipse overlap. What is the sum of the numbers inside the ellipse but not inside the square?

Inside the ellipse and outside the square: 4 (ellipse only) and 13 (circle and ellipse). Sum $=4+13=17$. 28 takes every number in the ellipse, 23 adds the 6 that is in the square too, and 9 adds the square-and-ellipse 5 instead of the 13.