Blinding is part of this, as the participants do not know whether they received the competitor's product.
Add the three angles and set them to 180
4x-13+15+x+18=180
5x+20=180
5x=160
x=32
Then plug in to get A and C
A=4*32-13
A=115
C=32+18
C=50
9x135 = 1,215
10x135 = 1,350
11x135 = 1,485
Therefore, your answer would be 10.
