there are 120 question, 1 mark for a correct answer, - 1/2 Mark for wrong answer, - 1/4 Mark for unattempted question. Of 5 mark will get, then how many are incorrect answer.

Answer 1

If Mark got a score of 5, then if x were right and y were wrong, and z left blank,
x+y+z = 120
x - y/2 - z/4 = 5
Since there are only two equations, there is no unique solution.
But, given that
y = 5(x-28)
z = 260-6x
You can see that x >= 28 and z <= 43
Now make a table starting with x=28 and see what y has to be.