If you got 55.6% on a test and that test is worth 80%
So first let us calculate the actual percentage that you scored.
The actual percentage is

So your actual score on the test is 69.5%
The grading system varies across countries and states
The traditional grading scale is given below
90% to 100% corresponds to A grade.
80% to 89% corresponds to B grade.
70% to 79% corresponds to C grade.
60% to 69% corresponds to D grade.
0% to 59% corresponds to F grade.
69.5% can be rounded off to 70%
Therefore, 69.5% corresponds to C grade.
(note: the above grade is not valid in case your school follows a different grading scale)