24・15 If 1 pint of paint is required to paint a 6-foot tall statue, how many pints of paint are needed to paint (the same thickness) 540 replicas of 1-foot tall statues?
(A) 90.
(B) 72.
(C) 45.
(D) 30.
(E) 15.
(24th American High School Mathematics Examination, 1973)
[Solution] The number of pints of paint $p$ is proportional to the product of the number of statues painted $n$ and the square of the height of each statue $h$, which means
$$
p=k n h^{2} \text {. }
$$

When $n=1, h=6, p=1$, we get $k=\frac{1}{36}$,
so $p=\frac{1}{36} n h^{2}$.
Then for $n=540, h=1$, we get $p=\frac{1}{36} \cdot 540 \cdot 1^{2}=15$. Therefore, the answer is $(E)$.