Finding the Inverse of a Rational Function

The function $C = \frac{20 + 0.4 n}{100 + n}$ represents the concentration $C$ of an acid solution after $n$ mL of 40% solution has been added to 100 mL of a 20% solution. First, find the inverse of the function; that is, find an expression for $n$ in terms of $C .$ Then use your result to determine how much of the 40% solution should be added so that the final mixture is a 35% solution.