G2.1 In Figure $1, A B C D$ is a rectangle, $A B=6 \mathrm{~cm}$ and $B C=$ $10 \mathrm{~cm} . M$ and $N$ are the midpoints of $A B$ and $D C$ respectively. If the area of the shaded region is $a \mathrm{~cm}^{2}$, find the value of $a$.
$a=\frac{1}{4}$ area of rectangle $=\frac{1}{4} \times 6 \times 10=15$