Adenine = A
Thymine = T
Guanine= G
Cytosine= C
A and T always pair together (so there are equal numbers of A and T)
G and C always pair togehter (so there are equal numbers of G and C).
20% A means there must also be 20%T.
20%+20%=40%.
100%-40%=60%
So that means that there is 60% C and G, and because there are equal numbers of both, you do 60% divided by 2, so get 30%.
Therefore, percentage of Guanine = 30 %
:)