A Laboratory Technician Records the Life Span (In Months) of Rats

Short Answer

A laboratory technician records the life span (in months) of rats treated at birth with a fertility hormone. From the following frequency distribution, find the sample standard deviation of life span.
  $$\begin{array} { | l | l | l | l | } \hline \text { Life span (months) } & \text { Midpoint } x & \text { Frequency } f & \text { Product } x f \\\hline -0.5 - 2.5 & & 11 & \\\hline 2.5 - 5.5 & & 20 & \\\hline 5.5 - 8.5 & & 21 & \\\hline 8.5 - 11.5 & & 29 & \\\hline 11.5 - 14.5 & & 19 & \\\hline\end{array}$$ Please round your answer to the nearest whole number, if necessary.
__________ months