In an examination of 100 marks a person attempted 77 questions and got 42 correct answer and 35 wrong answer. If each correct answer carried 2 marks and wrong answer carried 0.50 marks. How much total marks the person scored?

1 answer

In what system do you get .5 marks for a wrong answer?
Did you mean .5 marks are deducted for a wrong answer?

then what is 2(42) - 35(.5) ??

This question makes no sense.
If there are a total of 100 marks, and each correct answer is worth 2 marks, then there could only be a maximum of 50 questions.
How could the person answer 77 of them?

MathMate and I both answered a similar problem a few years ago:
http://www.jiskha.com/display.cgi?id=1417162742
After you find and fix your problem, follow this example.