If total amount of adenine in a ds DNA is 30% then what will be the percentage of purine and cytosine respectively?

As its a double stranded DNA, G is always complimentary with C and A is complementary with T. Hence percentage of G= %C, and % A= %T.

If Adenine is 30 % then Thymine will also be 30%, Guanine and cytosine will be 40% so cytosine will be 20 %. Percentage of purine will be adenine + guanine= 30 % + 20 %= 50 %