To solve this you just have to use a rule of three to solve the percentages, remember that the 100% will be the 70 students, and we solve each case separetly:
70 students= 100%
sixth grades= 40%
Sixth grades= (40*70)/100
Sixth graders= 28
70 students= 100%
seventh grades= 20%
seventh graders= (20*70)/100
Seventh graders=14
So if we have that the rest of the students are eighth graders, we just add up the sixth and seventh graders and withdraw them from the total:
$27.79. First multiply the percentage as a decimal out of 100. Subtract that number from the original amount. Do the same thing with the number you got from subtracting(multiply by percentage then subtract from starting amount(the new one)) you end up with $27.79
