1.106 Country $A$ has $c \%$ of the world's population and $d \%$ of the world's wealth, while country $B$ has $e \%$ of the world's population and $f \%$ of the world's wealth. Assuming that the citizens of country $A$ share the wealth of country $A$ equally, and the citizens of country $B$ share the wealth of country $B$ equally, find the ratio of the wealth of a citizen of country $A$ to that of a citizen of country $B$.
(A) $\frac{c d}{e f}$.
(B) $\frac{c e}{d f}$.
(C) $\frac{c f}{d e}$.
(D) $\frac{d e}{c f}$.
(E) $\frac{d f}{c e}$.
(44th American High School Mathematics Examination, 1993)
[Solution] Let $p$ be the world's population, and $w$ be the world's total wealth. Then $\frac{p c}{100}$ is the total number of citizens in country $A$, and $\frac{w d}{100}$ is their total wealth. Therefore, the wealth owned by each citizen of country $A$ is
$$
\frac{\frac{w d}{100}}{\frac{p c}{100}}=\frac{w d}{p c} .
$$

Similarly, the wealth owned by each citizen of country $B$ is $\frac{w f}{p e}$.
Therefore, the wealth ratio is
$$
\frac{\frac{w d}{p c}}{\frac{w f}{p e}}=\frac{d e}{c f} .
$$

Hence, the answer is (D).