If I get a 20% on my exam

If my mark before the exam was 53%, and I got a 20%, where exams are worth 30% of my grade, what would be my final mark?

there is no way to know that without knowing the two scores, exams and homework, unless you mean the 20 counted as 30 percent and the 53 as 70, which would be a weighted averages problem:
((.53)7+(.2)3)/10
that would make it a 22.26 percent

A failing grade.