If the arithmetic mean of the following frequency table is 50 , then
\begin{array}{|c|c|}
\hline Class & Frequency \\
\hline 0-20 & 17 \\
20-40 & f_1 \\
40-60 & 32 \\
60-80 & f_2 \\
80-100 & 19 \\
\hline & \mathbf{1 2 0} \\
\hline
\end{array}