Estimate the standard deviation of a bond's returns if it had the following annual returns over the past $3$ years: $-15\mathrm{.}00\%,2\mathrm{.}00\%,$ and $20\mathrm{.}00\%?($Round the value to $100$th decimal and Please enter the value only without converting it to a decimal format. If the answer is $8\mathrm{.}55\%,$ enter $8\mathrm{.}55)$