Calculate the standard deviation of the following set of returns:

Year $1$: $15\%,$ Year $2$: $21\%,$ Year $3$: $6\%,$ Year $4$: $-15\%,$ and Year $5$: $9\%$

$($Do not round intermediate calculations. Enter your standard deviation answer as a percent rounded to $2$ decimal places, e$.$g$.,12\mathrm{.}34.$ Do not input a percent sign with your answer.$)$