[tex] \frac{n+5}{5} = \frac{n}{5} + 1 [/tex]
This is true because
[tex] \frac{n+5}{5} = \frac{n}{5} + \frac{5}{5} [/tex]
So...
[tex] \frac{f+9}{9} = \frac{f}{9} + 1 [/tex]
The two one's cancel each other out, leaving us with
[tex] \frac{n}{5} = \frac{f}{9} [/tex]
