You have $20 \%$ of adenine. If you have $20 \%$ of adenine then you have $20 \%$ of thymine, because the amount of adenine and thymine is equal.
So, Adenine $+$ Thymine $=40 \%$
Therefore,the percentage of the amount of Guanine and Cytosine $=100-40 \rightarrow 60 \%$ i.e., $30 \%$ of Guanine and $30 \%$ of Cytosine, because their amounts are equal in the DNA stand. Hence the percentage of the bases of pyrimidines, i.e., Cytosine and Thymine $=30+20=$ $50 \%$