Allyson is taking a class in which the grade is determined by the mean (average) of four tests. She has the following test scores so far in the course: 68, 87, and 90 and asks you to help her figure out what her last test score must be to get an 85 in the class. She tells you that a particular formula is used to figure the final grade of each student in the class. Let the variable "x" equal the last test score. a) Write the formula needed to solve this problem.
b) Substitute the values and the variable into the formula.
c) Solve for the test score needed to get an overall grade of 85% in the class.