OK well you have to add three parts together so let's start by taking the midterm mark.
This is marked out of 50 and accounts for up to 30% of the final score.
So if you score 50/50 in the midterm then that would give you the full 30%.
If you got 25/50 then that would give you half the 30% allocation which is 15%.
What would you have to score out of 50 to get a third of the 30% allocation, i.e. 10%?
Try with a few other numbers and that should give you some idea of how to work it out as a formula.
The quizzes work the same way. There are three quizzes, each out of 10. If you get 10/10 for all three, that gets you the full 20%. What would you get if you scored 5/10 for all three? As with the midterm, play with some numbers and see if you can derive a formula. Don't just guess at a formula; work it out.
Here's some example data for you. Let's say you get 50/100 for the final, 25/50 for the midterm and 5/10 for all three quizzes. What would the overall grade be?