12. (5 points) A group of students participates in a tree-planting activity. If 1 girl and 2 boys form a group, there are 15 boys left over; if 1 girl and 3 boys form a group, there are 6 girls left over. Therefore, the number of boys participating in the tree-planting activity is $\qquad$, and the number of girls is $\qquad$.
【Analysis】According to the problem: If there are 6 more girls, there will be $3 \times 6=18$ fewer boys, which means each group will have $3-2=1$ more boys, and thus $15+18=33$ boys are needed. Based on this, we can find the number of groups, and then determine the number of boys and girls. Solve accordingly.

【Solution】 $(15+6 \times 3) \div(3-2)$
$$
\begin{array}{l}
=(15+18) \div 1 \\
=33 \div 1 \\
=33 \text { (groups); } \\
33 \times 1=33 \text { (names) } \\
33 \times 2+15 \\
=66+15 \\
=81 \text { (names); }
\end{array}
$$

Answer: There are 81 boys and 33 girls.
The answer is: 81,33.
【Comment】The difficulty of this problem lies in understanding that 6 more girls means $3 \times 6=18$ fewer boys.