6. The large rectangle shown is made up of nine identical rectangles whose longest sides are $10 \mathrm{~cm}$ long. What is the perimeter of the large rectangle?
A $40 \mathrm{~cm}$
B $48 \mathrm{~cm}$
C $76 \mathrm{~cm}$
D $81 \mathrm{~cm}$
E $90 \mathrm{~cm}$
6. C Since the horizontal lengths of the small rectangles are $10 \mathrm{~cm}$, the length of the large rectangle is $2 \times 10 \mathrm{~cm}=20 \mathrm{~cm}$. Also, since the sum of the heights of five small rectangles is equal to the length of the large rectangle, the height of a small rectangle is $20 \mathrm{~cm} \div 5=4 \mathrm{~cm}$. Therefore, the perimeter of the large rectangle is $(2 \times 20+2 \times 10+4 \times 4) \mathrm{cm}=76 \mathrm{~cm}$.