According to Chargaff's rule, [A] + [G] = [C] +[T]
[A]+[G]+[C]+[T] = 100 %
Adenine [A] is paired with Thymine [T], A=T
Cytosine [C} is paired to Guanine [G], G =C
Here,[T] 40 % therefore, % of [A] is also 40%,
Therefore; [G]+[C] = 100% - 80% = 20%
Hence; [G] = 10% and [C] = 10%
Therefore, the percentage of guanine present is 10%