A 55 can significantly lower your grade, especially if it's a major assignment or test. If the weight is high, it could drop your grade by a full letter or more.
Example 1:
Current grade: 90
Test weight: 20%
Score: 55
→ New grade: ~83
Example 2:
Current grade: 85
Test weight: 30%
Score: 55
→ New grade: ~76