2.5 A can run a circular track in 40 seconds, B runs in the opposite direction, and meets A every 15 seconds, how many seconds does it take for B to run the track?
(A) $12 \frac{1}{2}$.
(B) 24 .
(C) 25 .
(D) $27 \frac{1}{2}$.
(E) 55 .
(10th American High School Mathematics Examination, 1959)
[Solution 1] Let the time it takes for B to run one lap be $t$ seconds. According to the problem, we have
$$
\frac{15}{40-15}=\frac{t}{40} \text {, }
$$

Solving this, we get $t=24$.
Therefore, the answer is $(B)$.
[Solution 2] Let the time it takes for B to run one lap be $t$ seconds, and the length of the track be $l$. Then, according to the problem, we have
$$
\frac{l}{40} \cdot 15+\frac{l}{t} \cdot 15=l \text {, }
$$

which simplifies to $\frac{3}{8}+\frac{15}{t}=1$, yielding $t=24$.
Therefore, the answer is $(B)$.