$\begin{array}{l}
a){R_{t{\rm{d}}}} = {R_1} + {R_2} = 20 + 30 = 50\\
I = \frac{U}{{{R_{td}}}} = \frac{{15}}{{50}} = 0,3(A)\\
b){I_1} = {I_2} = I = 0,3(A)\\
{U_1} = {I_1}.{R_1} = 0,3.20 = 6(V)\\
{U_2} = {I_2}.{R_2} = 0,3.30 = 9(V)\\
c)I' = 0,6\\
\Rightarrow {R_{TD}}' = \frac{U}{{I'}} =...