75x+( 100-x)* 83 = 100 * 8 1

8x=8300-8 100

X=25 people

There are 25 boys and 75 girls.

2. There are x adults and 100-x children.

2x+ 100-x= 1 10

x= 10

Adults 10, 90 children.

3, 85*5-83*4=93 points

She got at least 93 points in the fifth exam.

4. 142 * 6- 14 1 * 5 = 147cm。

The height of the sixth person is 147 cm.