According to Chargaff's rule, for a double stranded DNA, $[A] = [T]$. Since $[A] = 30\%$, then $[T] = 30\%$. Since $[C] = [G]$, then $100 - [A + T] = 100 - [30 + 30] = 100 - 60 = 40\%$. Thus, $C = G = 20\%$ each. Therefore, $[A] = 30\%, [T] = 30\%, [G] = 20\%, [C] = 20\%$.