Grade Possibility?

Hi! There are two similar questions here:

This is a math problem related to my grade that I was wondering about:

So, our math final exam is worth 20% of our final grade. There are three terms in a year though. The three term's average is 80% of your final grade. So, the average of my three terms is 75%. If I get and A (86% or 52/60) on my final exam, what will my final grade be?

Secondly:

I want to know how badly my mark can be affected if the following happens. (This is just for one term, so there is no weighing or final exams etc..)

I had a 90% out of 120 marks in science and got an 87% on a test out of 80 marks, because of this, my mark went down by 1%. Now, I am at an 89%. If in the next quiz I get 6/20, what will my mark be in science?

I hope the questions make sense, and would be grateful for any explanation or answer, so that I can plan accordingly.

Thanks in advance :)