math
posted by Jennifer .
Jane is taking a class and asks you to help her figure out what her last grade must be in the class to get a 85 in the class. She tells you the following calculation is used to find your final grade:
75+85+90+X
___________ >
4
x is her last score which has not been taken. Explain this equation and the process you would use to solve for the unknown test score. Solve for the test score.

4 X 85 = 340
75+85+90+ X= 340
250 + X= 340
X=90