to find the mean you add up all the numbers, then divide by how many numbers there is.
so for 6th grade you would add up 2+3+3+3+3+4+4+5+5+5+5+6+6+6+6+6+7+9+10+10=?
and then divide that by 20.
same with 8th grade.
and then you would subtract one from the other
