400
If a strand of DNA contains 20% adenine, it also contains this % of cytosine
What is 30%?
The % of A and % of T are equal, since they base pair, so 20 + 20 =40% A + T
A + T + G + C adds up to 100%, so if A+T =40%, then G+C must equal 60%, or 30% C (and also 30% G)