20. The diagram shows a pyramid made up of 30 cubes, each measuring $1 \mathrm{~m} \times 1 \mathrm{~m} \times 1 \mathrm{~m}$.
What is the total surface area of the whole pyramid (including its base)?
A $30 \mathrm{~m}^{2}$
B $62 \mathrm{~m}^{2}$
C $72 \mathrm{~m}^{2}$
D $152 \mathrm{~m}^{2}$
E $180 \mathrm{~m}^{2}$
20. C When the pyramid is viewed from above, it can be seen that the total area of the horizontal part of the surface of the pyramid (excluding its base) is the same as that of a square of side 4 metres, that is $16 \mathrm{~m}^{2}$. The area of the base of the pyramid is also $16 \mathrm{~m}^{2}$. Finally the total area of the vertical part of the pyramid is equal to $(4 \times 1+4 \times 2+4 \times 3+4 \times 4) \mathrm{m}^{2}=40 \mathrm{~m}^{2}$. So the total surface area of the pyramid is $(16+16+40) \mathrm{m}^{2}=72 \mathrm{~m}^{2}$.