17-53 Two poles are 20 and 80 high, respectively, and are 100 apart. The lines connecting the top of each pole to the bottom of the other intersect at a height of
(A) 50.
(B) 40.
(C) 16.
(D) 60.
(E) None of the above.
(2nd American High School Mathematics Examination, 1951)
［Solution］As shown in the figure, by the property of similar triangles, we can get
$$
\frac{1}{x}=\frac{1}{20}+\frac{1}{80} \text {, }
$$

Therefore,
$$
x=16 \text {. }
$$

So the answer is $(C)$.