if my grade is an 81% and i get a 51% on a test worth 10% what's my grade now
If your grade is currently at 81% and you get a 51% on a test worth 10%, you would add the weighted score of the test to your current grade.
0.10 * 51 = 5.1
81 + 5.1 = 86.1%
So, your new grade would be 86.1%.