HELP!!! Can someone explain to me...if a double stranded DNA virus has a 40 % G + C content, then?

Question

how much A will be present? The book states the correct answer is 30%, but i do not understand....

Answer 1

If 40% of the bases are G+C, then 60% must be A+T.

Since the virus is double stranded DNA, then A must equal T, so each must be 30%.

That help?

Answer 2

A double bonds to T
C triple bonds to G

you need that first of all. After that it's math, if C and G TOGETHER equal 40% of the DNA then C = 20% and G = 20%

100% - 40% = 60%

Since the other pair is A and T, 60 divided by two gives you A = 30% and T = 30%