50%
1) Gathering the data
12 are 6th graders
18 7th graders
6 8th graders
2) Let's add them up at first to get the whole number of students who tutor:
12 +18 +6 = 24 +12 = 36
So we can say that
36 -------------- 100%
The 7th graders: 18 students
So we can write a proportion for that
36-------100%
18 ------- x
36x = 1800 Divide both sides by 36
x =50
3) So the answer is 50% of them are 7th graders.