If you have a grade of 54.96 and you take a summative test worth 80%, it would bring up your grade by the following calculation:
New grade = (54.96) + (80/100) * (100 - 54.96)
New grade = 54.96 + (0.8) * (45.04)
New grade = 54.96 + 36.032
New grade = 90.992
So, your new grade would be 90.992
It's worth noting that this calculation assumes that the summative test is worth 80% of your overall grade, and that your previous grade was 54.96 out of 100. If this isn't the case, the result will be different.