2.75 A team is 6 kilometers long and advances at a speed of 5 kilometers per hour. A communicator rides a horse from the head to the tail of the team to deliver a message, and then immediately returns to the head of the team, taking a total of half an hour. What is the speed of the communicator in kilometers per hour?
(A) 25.
(B) 24.
(C) 20.
(D) 18.
(2nd "Five Sheep Cup" Junior High School Mathematics Competition, 1990)
[Solution] Let the speed of the messenger be $x$ km/h. The relative speed of the messenger to the team is $(x+5)$ km/h when moving from the front to the back, and $(x-5)$ km/h when moving from the back to the front. According to the problem,

i.e., $\square$
$$
\frac{6}{x+5}+\frac{6}{x-5}=\frac{1}{2} \text {, }
$$

Solving this, we get
$$
x^{2}-24 x-25=0 \text {. }
$$

Therefore, the answer is $(A)$.