$20 \cdot 111$ Draw three circles on the same side of a straight line: one circle is tangent to the line with a radius of 4, the other two circles are equal, and each is tangent to the line and the other two circles. The radius of the two equal circles is
(A) 24.
(B) 20.
(C) 18.
(D) 16.
(E) 12.
(10th American High School Mathematics Examination, 1959)
[Solution] Let the radii of the two equal circles both be $x$. As shown in the figure, by the Pythagorean theorem, we have
$$
x^{2}+(x-4)^{2}=(x+4)^{2} .
$$

Solving this, we get $x=16$ or 0 (discard 0).
Therefore, the answer is $(D)$.