Suppose a stock has generated the following annual returns: $13\mathrm{.}9\%,-12\mathrm{.}9\%$ and $7\mathrm{.}9\%.$ What was the standard deviation of its returns? Answer in percent, rounded to two decimal places $($e$.$g$.,4\mathrm{.}32\%=4\mathrm{.}32).$