Help me in solving LBCV203A problem

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Learning course: C++ for problem solving - 1
Problem Link: CodeChef: Practical coding for everyone

question states that u r given the marks of the chef u have to find the minimum negative score he can get to achieve that score .
like for exmple i have received 16 marks and i know for each question i will get 3 marks s i will give 6 correct answer which will give me 18 marks and then 2 incorrect answer which will give me -2 marks so total is 18-2=16 .
Thus for this case answer will be 2 .