1・115 A person ordered 4 pairs of black socks and some blue socks, with the price of black socks per pair being twice that of blue socks. However, the order form had the colors filled in the wrong way, resulting in a 50% increase in expenditure. Therefore, the original planned ratio of the number of pairs of black socks to blue socks is
(A) $4: 1$.
(B) $2: 1$.
(C) $1: 4$.
(D) $1: 2$.
(E) $1: 8$.
(1st American High School Mathematics Examination, 1950)
[Solution] Let there be $x$ pairs of blue socks, each priced at $y$, obviously the price of each pair of black socks is $2y$. According to the problem, we have
$$
x \cdot 2 y + 4 \cdot y = \frac{3}{2}(4 \cdot 2 y + x \cdot y)
$$

Solving this, we get $x=16$, and $\frac{4}{x}=\frac{1}{4}$, which means $4: x = 1: 4$. Therefore, the answer is $(C)$.