Answer:
First case:
C -> 30% and T -> 20%
Second case:
C -> 15% and T -> 35%
Explanation:
First of all you should know that if it is DNA, it's formed by two strands. Also, in DNA, the amount of A is the same as the amount of T. And the amount of C is the same amount of G.
So, if you have 20% of A, you will also have 20% of T.
And if you have 30% of G you will have 30% of C
I hope it's useful and that you understand it!
PD: If there are any mistakes in grammar or spelling it's because my native is Spanish, not English.
