12. The shape in the diagram is made up of a rectangle, a square and an equilateral triangle, all of which have the same perimeter. The length of the side of the square is $9 \mathrm{~cm}$. What is the length of the shorter sides of the rectangle?
A $4 \mathrm{~cm}$
B $5 \mathrm{~cm}$
C $6 \mathrm{~cm}$
D $7 \mathrm{~cm}$
E $8 \mathrm{~cm}$
12. C The perimeter of the square is $4 \times 9 \mathrm{~cm}=36 \mathrm{~cm}$. Therefore, since the perimeter of the square and the equilateral triangle are the same, the side-length of the equilateral triangle is $36 \mathrm{~cm} \div 3=12 \mathrm{~cm}$. Hence the length of each of the longer sides of the rectangle is $12 \mathrm{~cm}$. Since the perimeter of the rectangle is also $36 \mathrm{~cm}$, the length of each of the shorter sides of the rectangle is $(36-2 \times 12) \mathrm{cm} \div 2=6 \mathrm{~cm}$.