Need to solve for k in the following formula $\displaystyle M = M_0 2^{ - kt}$

given that Mo reduces by a factor of 16 in 32 days, the what factor will it reduce by in 10days.

I get as far as this

$\displaystyle

\begin{array}{l}

\frac{1}{{16}}M_0 = M_0 2^{ - kt} \\

\frac{1}{{16}} = 2^{ - kt} \\

\end{array}

$

I think i need to use logs here to solve, but I'm not sure how, as i would be base 2 I think