2・7 A river boat travels at a constant speed from point A to point B. Along the riverbank, there is a road, and the captain notices that a bus passes from behind every 30 minutes, while a bus comes from the opposite direction every 10 minutes. Assuming the buses operate between points A and B with uniform departure intervals and travel at a constant speed, the time interval between each bus departure is (minutes)
(A) 12.
(B) 15.
(C) 18.
(D) 20.
(3rd "Five Sheep Cup" Junior High School Mathematics Competition, 1991)
[Solution] Let the interval between each bus departure be $x$ minutes.
When the boat is moving, after the front car meets the boat, the next car in the same direction needs an additional $30-x$ minutes to catch up with the boat, while the next car in the opposite direction can meet the boat $x-10$ minutes earlier.

Then the speed of the boat is $\frac{30-x}{30}$ times the speed of the car in the same direction, and $\frac{x-10}{10}$ times the speed of the car in the opposite direction. Since the car speed is constant,
we have $\frac{30-x}{30}=\frac{x-10}{10}$, solving for $x$ gives $x=15$.
Therefore, the answer is $(B)$.