Dear ABC

As per as given data which is

+2 marks for correct answer

-0.5 marks for incorrect answer

Calculation is given below

Total marks latika scored = 30 marks.

She got 20 questions correct means 20× 2 ( for each correct answer) = 40 marks.

Now she has got only total of 30 marks means 40-30 = 10 marks she have done negative marking.

Now total negative marking scored is 10 marks and for each -0.5 marks means 10/0.5 = 20 which means she attempted 20 questions as incorrect answer.

So your answer is 20 incorrect answer.

She have done 20 correct answer and 20 incorrect answer to score total of 30 marks according to your marking scheme.

Hope this helps you

Good luck