6. 6th grade- go to the two columns above $600, which is about 8 students in the first column and 5 students in the 2nd. 8 + 5 = 13, so 13 students in 6th grade earned $600 or more.
7th grade- again, go to the columns above $600, which is about 7 students in the first column and about 3 in the 2nd column. 7 + 3 = 10, so 10 students in 7th grade earned $600 or more.
Do the same thing for #7, just follow the rows exactly as #6.