What will my mark be after my exam?

So im in grade 10 accademic science and i find it challenging, my exam is in 2 days. I am pretty stressed about it. So can someone help me out with a question: if i have a 63 going into the exam and the exam is worth 20% of my final grade, what is the lowest mark i can get on the exam and still pass (over 50%) im sure there is a way to figure this out mathematically but i dont know how :p. Thanks A lot in advance i appreciate it.