Since the question asks the person this, what is the probability a club member is NOT a 7th grader. This means we have to find the probability of not being a 7th grader. the probability of it being a 7th grader is 14/40. This mean I subtract 14 from 40 to get 26. You can also add the probabilities from the 6th graders and 8th grades to also get 26/40.
Edit: Now that I just saw the simplifying of the previous answer I just realized that I didn't simplify. 13/20 is simplified. Props to the previous answer for doing what I did not.
