$a)$

$N = 120 \cdot 20 = 2400$

$2A + 3G = 2900$

$2A + 2G = 2400$

$G = 2900 - 2400 = 500$

$C = 500$

$A = \frac{2400 - 2 \cdot 500}{2} = 700$

$T = 700$

$N_1 = 1200$

$N_2 = 1200$

$T_2 = 400$

$A_1 = 400$

$A_2 = 700 - 400 = 300$

$T_1 = 300$

$C_2 = 10\% \cdot 1200 = 120$

$G_1 = 120$

$G_2 = 500 - 120 = 380$

$C_1 = 380$

$b)$

$L = \frac{2400}{2} \cdot 3,4 = 4080 \mathring{A}$

$LKHT = 2400 - 2 = 2398$