What is 18 = n -18 because i don't understand the question

Arestaurant manager wanted to get a better understanding of the tips her employees earn, so she decided to record the number of patrons her restaurant receives over the course of a week, as well as how many of those patrons left tips of at least 15%. the data she collected is in the table below. day mon tue wed thu fri sat sun patrons 126 106 103 126 153 165 137 tippers 82 87 93 68 91 83 64 which day of the week has the lowest experimental probability of patrons tipping at least 15%? a. sunday b. saturday c. friday d. thursday