11. A picture, together with its frame, makes a square of side-length $80 \mathrm{~cm}$. The frame is $4 \mathrm{~cm}$ wide.
What percentage of the area of the square is covered by the frame?
A $15 \%$
B $18 \%$
C $19 \%$
D $20 \%$
E $24 \%$
11. C The horizontal part of the frame consists of two rectangles, each measuring $80 \mathrm{~cm}$ by $4 \mathrm{~cm}$. The vertical part of the frame consists of two rectangles, each measuring $72 \mathrm{~cm}$ by $4 \mathrm{~cm}$. Therefore the percentage of the area of the square covered by the frame is
$$
\frac{2(80 \times 4+72 \times 4)}{80 \times 80} \times 100 \%=\frac{8(80+72)}{8 \times 8} \%=\frac{80+72}{8} \%=(10+9) \%=19 \% .
$$