Final round of math... [EDIT]

So these are the last math questions for now. They require a decent amount of skill, but it's not "OMFG WTF is this!!! /ragequit" stuff. It's not impossible, and these aren't the hardest questions ever. They provide some degree of challenge, but if you're looking for something only you and 10 other people in the world can solve, you're in the wrong place. It's still high school stuff.

I've re-read the questions a couple of times, very carefully, so I hope nothing has slipped past, but you never know...

If the questions aren't clear or worded poorly, please let me know so I can edit it here.

So, I guess I can only wish you good luck, and I hope you have some fun! :)

The giveaway will run until Wednesday, 6pm GMT+1. Contribution of $30.01 needed.

Edit:
New rule: let me see your stuff on paper. No using online equation solvers and stuff. You'll have to be able to tell me what you did. Exams are coming early this year! :)