A worked example: 68 marks out of 80
Dividing 68 by 80 gives 0.85, and multiplying by 100 gives a percentage of 85% — a letter grade of B on the common A/B/C/D/F scale (90/80/70/60 cutoffs).
The formula: marks obtained divided by total marks, times 100
Percentage = (marks obtained ÷ total marks) × 100. With 156 marks obtained out of a total of 200, that's (156 ÷ 200) × 100 = 78% — a letter grade of C, just two points inside the C band rather than crossing into B territory at 80%.
A failing result: 90 marks out of 200
The same formula applies below the passing threshold just as it does above it: (90 ÷ 200) × 100 = 45%, which falls under the 60% cutoff and is graded F. The formula itself doesn't distinguish "failing" from "passing" — it always just divides and multiplies — the letter grade bands are what turn that raw percentage into a pass/fail-style judgment.
Why the total marks value matters as much as the raw score
A raw score alone is meaningless without knowing what it was out of — 68 marks is a B when the total is 80, but would be a completely different percentage and grade against a total of 100 or 68. Converting to a percentage first is what makes scores from tests with different maximum marks comparable to each other at all.