DNA base pairing adheres to Chargaff’s rules: adenine (A) pairs with thymine (T), and guanine (G) pairs with cytosine (C). Consequently, the percentage of adenine equals that of thymine (\( A = T \)), and the percentage of guanine equals that of cytosine (\( G = C \)). The sum of all base percentages is 100%:
\[A + T + G + C = 100\%\] If the adenine percentage is 30%:
\[A = 30\%, \quad T = 30\% \quad (\text{as } A = T)\] \[A + T = 30\% + 30\% = 60\%\] The remaining percentage comprises guanine and cytosine:
\[G + C = 100\% - 60\% = 40\%\] Given that \( G = C \):
\[G = C = \frac{40\%}{2} = 20\%\] Therefore, the percentage of cytosine (C) is 20%.