$2 \cdot 3$ If a merchant's cost of goods decreases by $8 \%$, and the selling price remains unchanged, then his profit (determined by the cost) can increase from the current $x \%$ to $(x+10) \%, x \%$ is
(A) $12 \%$.
(B) $15 \%$.
(C) $30 \%$.
(D) $50 \%$.
(E) $75 \%$.
(23rd American High School Mathematics Examination, 1972)
[Solution] Let $C$ be the current purchase price, then $0.92 C$ is the discounted price. $x$ is the current profit, expressed as a percentage. Then write the fixed selling price equation, we get

that is
$$
C(1+0.01 x)=0.92 C[1+0.01(x+10)],
$$

we have
$$
0.08(0.01 x)=0.92 \cdot 1.1-1=0.012 \text {. }
$$

Therefore, the answer is $(B)$.