$100(1-\frac{12 \cdot 0.85^{a+1}}{12 \cdot 0.85^a})$
As seen here(Or pic below), this evaluates to 15.
There is one step I am worried is wrong, can I just rewrite the exponentiations over like this?
$100(1-\frac{12 \cdot (a+1) \cdot \log 0.85}{12 \cdot a \cdot \log 0.85})$
Because in the end I end up with the following
$\frac{100}{a}$
Instead of 15.