So you have 16.72% of your total grade guaranteed. You can verify this because you know that .2 (20%) is one fifth of your total grade (.2*5 = 1 or 20%*5 =100%, or 100/5=20). So to verify, you assume you got the same percentage for each of the other 4 segments of 20%, or a total of 5 segments of 20% where you got 83.64%. So multiply .1672 * 5 = .836 (or 16.72% * 5 = 83.6%)
Because it is merely a percentage, if you got the same percentage on every grade, no matter what it was weighted, it would total that percentage, you can see that if you got 83.64% on every grade, it should equal 83.64% for the total consolidated grades, and when we check our problem this is true, so we know we did it correctly.
Another way to check (and now that I think about it, this is also a better way because it allows for unusual weights such as a test which is worth 13.4938% or any other number) would be to say you got 83.64% on the other 80%, and 20%+80% = 100% so (83.64%*20% + 83.64%*80% = 83.64%*100%=83.64%) and plug in the numbers ( .8364*.2 + .8364*.8 = .8364*1 = .8364)
So we can see, again, that it is done correctly. All this means that you just multiply the percentage you received by the weight (if the weight is given as a percentage of the total)
So you have about 16.72% locked in for sure, because 16.72 is 83.64% of 20, which is 20% of 100. And if you maintain the same average, once all the scores are added together, you will have an 83.64% average for the course.