DNA has four type of nitrogenous bases in its nucleotides named as adenine, thiamine, cytosine and guanine and the abbreviations for these bases are A, T, C and G respectively. DNA is a double stranded bio molecule and the nucleotides of both the strands of DNA are joined with each other with the help of either double bonds or triple bonds which exist between the nitrogenous bases of its nucleotides. There are two type of bases, purines and pyrimidines. Adenine and guanine are purine bases while cytosine and thiamine are pyrimidine bases.
Chargaff’s base pair rule says that the ratio of purine bases with respect to pyrimidine bases must be 1:1 ratio. It can be possible only if a purine base pairs with pyrimidine base of the adjacent DNA helix. So, adenine which is a purine always pairs up with thiamine which is a pyrimidine with the help of two hydrogen bonds while guanine being a purine always pairs up with cytosine which is a pyrimidine with the help of three hydrogen bonds. It simply means that a DNA molecule will have as many adenine as many there are thiamine. Also, for the same reason there will be as many cytosine as many there are guanine.
So, 20% of T means same amount of A which is 20%. Hence, A and T will together form 20% + 20% = 40% of the sample.
In the remaining 60% sample the amount of C and G is equal that means it will have 30% C and 30% G.
So, the answer is 30% C or cytosine.