There are a total of 12 chocolate chip + 10 sugar + 18 peanut butter = 40 cookies in the sack. The probability of selecting a cookie that is not a peanut butter cookie is (12 chocolate chip + 10 sugar) / 40 = 22/40 or 0.55. So, there is a 55% chance that the selected cookie will not be a peanut butter cookie.