The Following Data Represent the Ages (In Years) of a Sample

The following data represent the ages (in years) of a sample of 25 employees from a government
department: $\begin{array}{|l|l|l|l|l|l|l|l|l|l|}\hline 31 & 43 & 56 & 23 & 49 & 42 & 33 & 61 & 44 & 28 \\\hline 48 & 38 & 44 & 35 & 40 & 64 & 52 & 42 & 47 & 39 \\\hline 53 & 27 & 36 & 35 & 20 & & & & & \\\hline\end{array}$ The frequency distribution for the data, using five class intervals and the value 20 as the lower limit of the first class is given below. $$\begin{array} { c c } \hline \text { Class Limits } & \text { Frequency } \\\hline 20 \text { up to } 30 & 4 \\30 \text { up to } 40 & 7 \\40 \text { up to } 50 & 9 \\50 \text { up to } 60 & 3 \\60 \text { up to } 70 & 2 \\\text { Total } & 25 \\\hline\end{array}$$ Find the mean of the grouped data.