In the figure a square, a circle and an ellipse overlap. Add the numbers that lie in exactly one figure and subtract the number that lies in all three. What is the result?

Exactly one figure: 3 (square), 8 (circle) and 5 (ellipse), sum 16. The number in all three is 2, so the result is $16-2=14$. 21 also adds the 7 outside every figure, 16 forgets to subtract, and 12 subtracts the 2 twice.