100 + 0.50c = 1.50c <== ur equation
100 = 1.50c - 0.50c
100 = c....so he would have to sell 100 cups of coffee to break even
I used the completing the square method to get the (x+2)^2. The final line is in standard form.
4.65
13 ÷ 20 is 0.65, just add the 4