If 22% are sixth graders, then the rest must not sixths graders.
100% of the school - 22% sixth grades = 78% are not sixths graders
The sum of angle AOB and angle BOC is equal tot he angle of AOC so
6x+5 +4x-2 = 8x+21
10x+3=8x+21
Subtract three and 8x from both sides
2x=18
Divide by two
x=9
Hope this helped!
0.074n = 74
multiply both sides by 1000 to make the process simpler
74n = 74000
divide both sides by 74
n = 1000