A. 20
Begin by substituting the known value. I have removed the symbols for now just to make the equation easier to read. 55 = 3.5x - 15
Add 15 on both sides of the equation to cancel out the subtraction. 70 = 3.5x
Finally, divide both sides by 3.5 to cancel out the subtraction. 20 = x
So, Starla bought 20 napkins.