Erwin Chargaff's rule for double-stranded DNA states that the ratios between Adenine and Thymine and between Guanine and Cytosine are constant and equal to one . This implies that the molar amount of Adenine is always equal to Thymine (%A = %T) and the amount of Guanine is equal to Cytosine (%G = %C) . Given that Thymine is 20%, Adenine must also be 20%, resulting in a total of 40% for A-T base pairs . The remaining 60% of the DNA composition (100% - 40%) must be divided equally between Guanine and Cytosine . Therefore, the percentage of Guanine is 30% . Mastering these fundamental molecular calculations is a high-priority strategy, as NCERT textbooks serve as the primary source for approximately 95% of NEET Biology questions .