3. Four identical small rectangles are put together to form a large rectangle as shown. The length of a shorter side of each small rectangle is $10 \mathrm{~cm}$. What is the length of a longer side of the large rectangle?
A $50 \mathrm{~cm}$
B $40 \mathrm{~cm}$
C $30 \mathrm{~cm}$
D $20 \mathrm{~cm}$
E $10 \mathrm{~cm}$
3. B From the diagram, the length of a small rectangle is twice the width. Hence the length of a small rectangle is $20 \mathrm{~cm}$. Therefore the length of the large rectangle, in $\mathrm{cm}$, is $20+2 \times 10=40$.