### Substitution

After some calculation I arrive to the following equation $\frac{D*p - D+ I}{D*~~(P-1)}$. ~~(P-1)}$.

Whichever be the command I use I cannot put it as $\frac{D*(p - 1)+ I}{D*(P-1)}$ or as $1-\frac{I}{D*(P-1)}$ or ~~$1-\frac{I}{D}\frac{1}{(P-1)}$. ~~$1-\frac{I}{D}\frac{1}{(P-1)}$.

This last equation is the one of interest since I would like to solve $1-\frac{I}{D}\frac{1}{(P-1)}==0$ according to ~~$\frac{D}{I}$. ~~$\frac{D}{I}$.

Is there a way to use `solve()`

acording to $\frac{D}{I}$ ? I have tried to substitue `x`

with a form of `substitute()`

for it, but it was an echec. I understand that substitue a ratio of 2 variables for one is a little bit complex.