10. Points $M$ and $N$ are the midpoints of two sides of the rectangle, shown in the diagram. What fraction of the rectangle's area is shaded?
A $\frac{1}{6}$
B $\frac{1}{5}$
C $\frac{1}{4}$
D $\frac{1}{3}$
E $\frac{1}{2}$
Solution
C

The diagram here shows all of the lower triangles reflected to be above the line $M N$. This makes clear that the total area of all of the shaded triangles is $\frac{1}{2} \times M N \times M L$. This area is half that of the rectangle $M N K L$, and so equals $\frac{1}{4}$ of the original rectangle.