Calculate the missing frequency from the following distribution, it being given that the median of the distribution is $24$.

$\begin{array}{|cccccc|}\hline \mathrm{Age} (\mathrm{in} \mathrm{years})& 0-10& 10-20& 20-30& 30-40& 40-50\\ \begin{array}{c}\mathrm{Number} \mathrm{of}\\ \mathrm{persons}\end{array}& 5& 25& ?& 18& 7\\ \hline\end{array}$

Which of the following is not a measure of central tendency?

If $x_i^{\text{'}}s$ are the mid points of the class interval of a grouped data, $f_i^{\text{'}}s$ are the corresponding frequencies and $\overline{x}$ is the mean then $\sum \left(f_i{x}_i-\overline{x}\right)=$?