Chargaff's rules govern base composition in double-stranded DNA and let us deduce unknown percentages from a single measured value. Because adenine always pairs with thymine and guanine always pairs with cytosine through specific hydrogen bonding, the mole percent of adenine equals that of thymine, and the mole percent of guanine equals that of cytosine. Here adenine is 30 percent, so thymine is also 30 percent, accounting together for 60 percent of all bases. The remaining bases must be guanine and cytosine, so their combined percentage is 100 minus 60, which equals 40 percent. Hence the answer is 40 percent. The choice of 30 percent wrongly assumes guanine alone equals adenine; 60 percent confuses the A plus T total with the G plus C total; and 20 percent comes from an incorrect halving. This applies the complementary base-pairing principle described in the NCERT Biomolecules chapter. As a consistency check, splitting the 40 percent equally gives guanine 20 percent and cytosine 20 percent, and the full set 30 + 30 + 20 + 20 sums to 100 percent, confirming the result is internally consistent with Chargaff's equalities.