If you're passionate about those subjects, then great! Take them
They say that for each hour you do inside of lessons, you should do an hour on your own, but that is somewhat unrealistic in my opinion.
I'd say that with maths, it depends how you well you understand each subject, but since you're doing FMSQ Add Maths, C1 and C2 should be a breeze, and there will be minimal new stuff, it will just be the applied unit which will be new, and generally they aren't too difficult, so you should do a past paper every so often and practice questions in your spare time, which shouldn't take too long if you just want to get an A (95+ UMS requires a bit more work, but is still relatively easy to achieve if you are good at maths). FMaths, is just two more applied units, depending on what you do the load may vary, and FP1 which I've only really just started, so I can't tell you much about that, but yet again, it's just practice the questions and do past papers, and you should easily get an A.
With the languages, you'll have to spend a lot of time learning the languages, and you need to practice them, but I don't really know much about taking them to A-Level, so I'm sure you'll understand better than I do anyway how you revise them, and what works best with you.
Chemistry, you just need to understand the content, which shouldn't be too bad with a good teacher, and considering your dad teaches A-Level Chem, you shouldn't have too many issues with not understanding anything, since you can always ask him. The right way to go about it, in my opinion, is to just learn the answers to every single possible question, even though the exams are changing, the syllabus hasn't changed much, and anything that has been added, was removed in the previous system, so they can't ask you for anything new really, and the answers don't change (e.g. I know that if I see a question asking about a general trend in something, most of the time, I need to mention shielding, I also know where to get all the marks, and how to answer them, but it helps to know the trends in answers, also). Using the mark schemes, I have created flash cards which contain all the possible answers for the majority of questions. Anything else, I'll use my understanding of both the content and the mark schemes to answer.
I think with economics, you need to generally have a good grasp of the content (same for all subjects really), and also a knowledge of how to answer the essay questions, they have high mark essay questions, and I think it's possible to get a good sum of the marks with only as single paragraph if you now how to do them properly, but I'm not really certain, since I haven't had any experience with economics, although I do hear it can be quite difficult (although I hear that about everything
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