$6.50 x 10 = 65. (first 10 boxes)
$5.50 x 10 = 55. (next 10 boxes)
65 + 55 = 120. (the 20 boxes added up)
183 - 120 = 63.
63 ÷ 4.50 = 14. (63 ÷ by the price for each remaining box, which is $4.50)
So, 10 + 10 = 20 (first 20 boxes) then + 14 because that is the remaining boxes of pens that you can buy with $183. This equals 34.
34 boxes of pens can be bought with $183.