In a test, +3 marks are given for every correct answer and -1 mark are given for every incorrect answer. Sona attempted all the questions and scored +20 marks, though she got 10 correct answers. How many incorrect answers has she attempted? (ii) How many questions were given in the test?

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.