mid-semester test

15 multiple choice questions, Each correct answer is worth 3 marks, and for each incorrect answer, 1 mark will be deducted instead.
function below:
Let no. of correct ans is m and incorrect is n, where m+n=15, m and n > =0
x be the sorce of this test, x= (3xm)-n

sorce below:
all correct : 45
1 wrong: 41
2 wrong: 37
3 wrong: 33
4 wrong: 29
5 wrong: 25
6 wrong: 21
7 wrong: 17
8 wrong: 13
9 wrong: 9
10 wrong: 5
11 wrong: 1
12 wrong: -3
13 wrong: -7
14 wrong: -11
all wrong: -15

if passing rate is 40% , I must get more than 18 marks in this test
therefore, less than 6 incorrect or more then 9 correct answers.
so, 60% correct answer will be pass in this test.