You're interested in knowing the GPA of each of your students in your English class. The class has 18 students and average GPA of 3.31. In order to find out the individual GPA's you ask each student for his/her GPA. Regretfully, on the day you asked only 15 students were present. How many of the remaining students must you ask for their GPA before being able to determine the GPA for all 18 students in the class?