x = number of questions
3y = points for a correct answer
-1x or -x = points for an incorrect answer
Adding up the correct and incorrect points/answers gives us the final score. We can solve to see how many questions were incorrect. (We are told 10 were correct)
3y + (-1x) = 20
3(10) + (-1x) = 20
30 - x = 20
10 = x
x = 10
So she got 10 correct and 10 incorrect, and there were 20 questions in all.