【Question 10】
A total of 100 students from grades three, four, and five participated in a tree planting activity, planting a total of 566 trees. It is known that the number of students in grades three and four is equal, with third-grade students planting an average of 4 trees per person, fourth-grade students planting an average of 5 trees per person, and fifth-grade students planting an average of 6.5 trees per person. The third-grade students planted $\qquad$ trees in total.
【Analysis and Solution】
(Method One)
If all 100 people were fifth-grade students, then they would plant $6.5 \times 100=650$ trees;
This is $650-566=84$ trees more than the actual number;
Replacing 2 fifth-grade students with 1 third-grade student and 1 fourth-grade student would result in $6.5 \times 2-4-5=4$ fewer trees;
The number of third-grade students is $84 \div 4=21$;
The third-grade students planted a total of $4 \times 21=84$ trees.
(Method Two)
Let the number of third and fourth-grade students be $x$ each, and the number of fifth-grade students be $y$;
$\left\{\begin{array}{c}2 x+y=100 \\ 4 x+5 x+6.5 y=566\end{array} ;\right.$ Solving this gives $\left\{\begin{array}{l}x=21 \\ y=58\end{array}\right.$;
The third-grade students planted a total of $4 \times 21=84$ trees.