By 1 / 4/5 do you mean (a) , (b) or (c) ? These options can be rendered in text as follows: (a) 1 4/5, (b) 1 / (4 / 5) and (c) (1 / 4) / 5 (maybe also adding a note would help).

Your equation has two variables: v and f, so at most you can express v through f.

LaTeX formulas have to be surrounded by [TEX]...[/TEX] tags (that's the rightmost button over the text area where you type your post, after you press "Preview Post" button).