15. (6 points) A motorcycle travels a 60-kilometer distance at an average speed of 20 kilometers per hour, and on the return trip, its average speed is 30 kilometers per hour. What is the average speed of the motorcycle for the entire round trip?
【Solution】Solution: $60 \times 2 \div(60 \div 20+60 \div 30)$
$$
\begin{array}{l}
=120 \div(3+2) \\
=120 \div 5 \\
=24 \text { (km/h) }
\end{array}
$$

Answer: The average speed of the motorcycle over the entire round trip is 24 km/h.