A girl of height $0.9\text{ m}$ is standing so that her distance from a lamp-post is $4.8\text{ m}$. If the lamp-post is $3.6\text{ m}$ high, what is the length of her shadow? 

The principle is that the large triangle formed by the lamp-post and its light ray is similar to the smaller triangle formed by the girl and her shadow. 1. Let the shadow length be $x$ m. Then the total distance from the lamp-post to the tip of the shadow is $4.8+x$. 2. By similarity of $\triangle ABE$ and $\triangle CDE$, $$\frac{BE}{DE}=\frac{AB}{CD}$$ so $$\frac{4.8+x}{x}=\frac{3.6}{0.9}=4.$$ 3. Hence $4.8+x=4x$, so $3x=4.8$, giving $x=1.6$. Option A and B come from incorrect algebra, while D would overestimate the shadow by not using proportional triangles correctly.