Topic: 90 sixth-grade students participated in the math competition, with an average score of 73 points, including 70 points for boys and 80 points for girls. How many boys and girls are there?

Solution: Suppose there are X boys and 90-x girls.

70x+80×(90-x)=73×90

70x+7200-80x=6570

7200- 10x=6570

10x=7200-6570

10x=630

x=630÷ 10

x=63

Number of girls =90-x=90-63=27.

A: There are 63 boys and 27 girls.