12. (6 points) In a parking lot, there are a total of 24 vehicles, among which cars have 4 wheels, and motorcycles have 3 wheels. These vehicles have a total of 86 wheels. How many three-wheeled motorcycles are there? $\qquad$ vehicles.
【Answer】Solution: Assume all 24 vehicles are 4-wheeled cars, then the number of tricycles is:
$$
\begin{aligned}
& (24 \times 4-86) \div(4-3), \\
= & 10 \div 1, \\
= & 10 \text { (vehicles), }
\end{aligned}
$$

Answer: There are 10 tricycles.
Therefore, the answer is: 10.