Number of cyphers after decimal before a significant figure start in [ Use :  ${\log }_{10}2=0.3010$ ]

Let  $N={\left(\frac{5}{2}\right)}^{-50}$
 $$\begin{gathered} \Rightarrow {\log }_{10}N=-50\times {\log }_{10}\left(\frac{5}{2}\right) \\ =-50({\log }_{10}\left(\frac{10}{2}\right)-{\log }_{10}2) \\ =-50(1-2{\log }_{10}2) \\ =-50(1-2\times 0.3010)=-50(1-0.6020) \\ =-50\left(.3980\right)=-19.9=-20+0.1=\overline{20}.10 \end{gathered}$$
$\therefore$  Number of zeros = 19