$36 \cdot 35 A$ and $B$ start moving at a uniform speed in opposite directions around a circular track from the ends of a diameter. If they start at the same time, they meet when $B$ has traveled 100 yards, and they also meet 60 yards before $A$ completes a lap, then the circumference of the track (in yards) is
(A) 400 .
(B) 440 .
(C) 480 .
(D) 560 .
(E) 880 .
(21st American High School Mathematics Examination, 1970)
[Solution] Let the circumference of the track be $2C$ yards, $A, B$ be the starting points, $F$ and $S$ be the points of the first and second meetings, respectively.
At the first meeting, $A, B$ have walked $C-100$ and 100 yards, respectively;
At the second meeting, $A, B$ have walked $2C-60$ and $C+60$ yards, respectively.
Since $A, B$ are moving at constant speeds, the ratio of the distances they travel in the same time remains constant, i.e.,
$$
\frac{C-100}{100}=\frac{2C-60}{C+60},
$$

Solving for $C$ gives $C=240$, thus $2C=480$. Therefore, the answer is $(C)$.