Couple points: do not change a variable like f to F; use * as multiplication sign, not x.

And use brackets to be CLEAR:

Upper bound F= 8.85 * 6.25 / (8.75 + 6.15)

lower bound F = 8.75 * 6.15/ (8.85 + 6.25)

Your question is unclear; can you post the EXACT original problem?

If there's any "mistake", your 2 equations should be:

Upper bound F= 8.85 * 6.25 / (8.85 + 6.25)

lower bound F = 8.75 * 6.15/ (8.75 + 6.15)

I've never seen 2 different values for a variable in the same equation