According to Chargaff's rule, the $DNA$ molecule should have an equal ratio of pyrimidine (cytosine and thymine) and purine (adenine and guanine). It means that the number of adenine molecules is equal to thymine molecules and the number of guanine molecules is equal to cytosine molecules. $\% \mathrm{A}=\% \mathrm{T}$ and $\% \mathrm{G}=\% \mathrm{C}$

If $dsDNA$ has $20 \%$ of cytosine, then according to the law, it would have $20 \%$ of guanine. 

Thus, percentage of $G+C$ content $=40 \%$

The remaining $60 \%$ represents both $\mathrm{A}+\mathrm{T}$ molecule.

since adenine and guanine are always present in equal numbers, the percentage of adenine molecule is $30 \%$.