18.57 If the base of a rectangle is increased by $10 \%$, and the area remains unchanged, then its height decreases by
(A) $9 \%$.
(B) $10 \%$.
(C) $11 \%$.
(D) $11 \frac{1}{9} \%$.
(E) $9 \frac{1}{11} \%$.
(3rd American High School Mathematics Examination, 1952)
［Solution］Let the base of the rectangle be $b$, and the height be $h$, the height is reduced by $x \%$. According to the problem, we have
$$
b h=(1+10 \%) b \cdot(1-x \%) h,
$$

we get $x=9 \frac{1}{11}$.
Therefore, the answer is $(E)$.