10. The diagram shows four congruent right-angled triangles inside a rectangle. What is the total area, in $\mathrm{cm}^{2}$, of the four triangles?
A 46
B 52
C 54
D 56
E 64
SOLUTION
$\mathbf{D}$

Let the lengths of the two shorter sides of the right-angled triangles be $a \mathrm{~cm}$ and $b \mathrm{~cm}$ as shown in the diagram. From the diagram in the question, it can be seen that $2 b=28$ and $a+2 b=30$, and hence $b=14$ and $a=2$. Therefore the total area, in $\mathrm{cm}^{2}$, of the four triangles is $4 \times\left(\frac{1}{2} \times 2 \times 14\right)=56$.