\(a,\) \(L=3,4.\dfrac{N}{2}=5100\left(nu\right)\)
\(b,\) \(C=\dfrac{N}{20}=150\left(ck\right)\)
\(c,\left\{{}\begin{matrix}2A+2G=3000\\2A+3G=3900\end{matrix}\right.\Rightarrow\left\{{}\begin{matrix}A=T=600\left(nu\right)\\G=X=900\left(nu\right)\end{matrix}\right.\)