Calculate standard deviation of marks secured by 100 examinees in the examination:
\begin{tabular}{|l|c|c|c|c|c|c|c|c|}
\hline Marks & \( 10-20 \) & \( 20-30 \) & \( 30-40 \) & \( 40-50 \) & \( 50-60 \) & \( 60-70 \) & \( 70-80 \) & \( 80-90 \) \\
\hline Number of Examinees & 19 & 3 & 2 & 49 & 24 & 2 & 0 & 1 \\
\hline
\end{tabular}
(Ans. \( \sigma=14.9 \) )