The magnification is given by
\[M=\frac{f_{PRIMARY}}{f_{SECONDARY}}\]
For a telescope of length
\[l\]
in normal adjustment, \[f_{PRIMARY}+f_{SECONDARY}=l\]
Suppose
\[m=50, \: l=M\]
, then \[50=\frac{f_{PRIMARY}}{f_{SECONDARY}}\]
and \[f_{PRIMARY}+f_{SECONDARY}=1m\]
From the first of these equations
\[f_{PRIMARY}=50 f_{SECONDARY}\]
and substituting this into the second gives \[50 f_{SECONDARY}+f_{SECONDARY}= 1 \rightarrow f_{SECONDARY}=1/51 \simeq 0.0196 m\]
then \[f_{PRIMARY}=50 f_{SECONDARY} \simeq 50 \times 0.0196 =0.98 m\]