the table shows the fractional number of students at each grade level who buy a school yearbook at Cold Springs Middle School. There are the same number of 7th grade students and 8th grade students who attend the school, but 50 fewer 6th grade students than 7th grade students. Of the 790 students in the school, how many buy a yearbook?
6th 7th 8th
Fraction of students who buy a yearbook 9/10 7/8 3/5
We can label x as the number of 7th and 8th graders, and x-50 as the number of 6th graders. So, we have x+x+x-50=3x-50=790 students. Therefore, 3x=840 students and x=280 students. We can solve for x-50, which is 280-50=230. Back again, we have 9/10(x-50)+7/8x+3/5x, which is just 9/10(230)+7/8(280)+3/5(280)=207+245+168=620 students.