If I get a 0 on 20% of my final grade...?

If you lose 20% of your grade, the highest you can possibly get in the cl is an 80%. You get an 80 if you get 100% of the possible points other than the 20% you're not doing. To get a C, you'll have to get at least an 87.5% on the rest of the cl, as you need a 70% out of 80% possible, or 7/8ths, which is 87.5%. If your grade is lower than an 87.5% without the 20% paper, then you won't get a C.