Final exam worth 25%

One quarter of the course is still open. The other 75% stays put.

80% now, goal 85%

needed = (85 − 80 × 0.75) / 0.25 = 100%

You need a perfect final. Scoring 100% produces 80 × 0.75 + 100 × 0.25 = 85%, which meets 85 and does not go past it. Open this 25% final.

A goal of 90 from the same 80 needs 120%. That is out of reach without extra credit. Compare 20% and 40%.

Open the final grade calculator.
Enter your current grade (80%), your goal (85%) and the final’s weight (25% of the course).
Read the score you need on the final at the top.
Optional: type a score in “If I score this on the final” (100% in this example) to see the course grade it would give you.