10.5#12&14 (The rules)

I don't know how to approach the problem.

and
I keep on getting this problem wrong. I don't know where I am making a
mistake.

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Well, it looks like you figured it out after awhile, and that's all to the good. In a lot of these situations people were just making an arithmetic mistake and fixed it on their own after a few minutes, which is good practice for the exam.

There is an issue I want to bring to your attention though, which is documenting your own due diligence when you asl for my help on a problem. I will have a much easier time helping you if I know what your already tried--and if I know that you did the basics like READ THE TEXTBOOK SECTION. I am often surprised how many people didn't pick that basic notion in the course of their education before now, and I don't want you to go through my class without the same misunderstanding. Moreover the ability and willingness to undertake the effort to gather the basic foundational skills is an important sign of professional maturity, and the profession will often be impatient or unkind if it judges you lacking in this regard.