Here, 

Given:

Number of primary windings ( \(\rm N_p\) )= \(\rm 550\) 

Number of secondary windings ( \(\rm N_s\))=  \(\rm ?\) 

Primary Voltage( \(\rm V_p\) )=  \(\rm 220V\) 

Secondary Voltage( \(\rm V_s\) )=   \(\rm 20V\) 

To find the secondary windings of the charger, we can use the transformer voltage formula:

$\frac{ N_p}{N_s}$ = $\frac{ V_p}{V_s}$

\(\rm or, \) $\frac{550 }{N_s}$ = $\frac{220}{20}$

\(\rm or, \) $\frac{ 550}{N_s}$ = $\frac{11 }{1}$

\(\rm or, \) $\frac{ 550}{11}$ = \(\rm N_s \)  

\(\rm or, \) \(\rm N_s \)   = \(\rm 50 \) 

The number of secondary windings of the charger is 50 turns.