Jake and mike took a different number of tests. jake scored 85 points on each test. mike's total score was 15 less than jake's total score. the following expression shows mike's total score:

85x − 15

in the expression, what does the variable x represent?
the total number of tests mike took
the total number of tests jake took
the sum of jake's and mike's total score
the difference between jake's and mike's total score