First, you know a triangle is equal to 180. Since you know the sum of the other two angles equal the outside angle, you can make them equal to each other, like 112=6x+3x+4. Combine like terms to 112=9x+4. Subtract 4 on both sides and divide by 9 on both sides to leave x by itself. This will leave you with x=12
The pricing of the boxes seems to be consistently 2.40 per box. Ethan's cost is .. 52.80 = 2.40 * (# of boxes) .. 52.80/2.40 = (# of boxes) .. 22 = # of boxes
