Answer: there were 40 questions on her math final.
Explanation:
Let x represent the total number of questions in the math test.
Mary got 85% correct on her math test. This means that the number of questions that she got right is
85/100 × x = 0.85 × x = 0.85x
This also means that the number of questions that she missed would be
x - 0.85x = 0.15x
Therefore, if Mary missed 6 questions, it means that
0.15x = 6
Dividing both sides of the equation by 0.15, it becomes
0.15x/0.15 = 6/0.15
x = 40