understand : $3.50 per box , she has already $35 worth of cookies which obviously she has 10 boxes.
plan: you will need to divide 200 by 3.5
solve: 200 divided by 3.5 = 57.142 ( we can't have any decimal points since she sells the boxes in a whole so we will say that she will have to sell around 57 boxes ) so the answer is 57