$解：(1)I_{1 }=\frac {U_{ }}{R_{1 }}=\frac {{ 6 }\ \text {V}}{{ 5 }Ω}={ 1.2 }\ \text {A}$
$(2)I_2=I-I_1=2\ \text {A}-1.2\ \text {A}=0.8\ \text {A}$
$R_{ 2 }=\frac {U_{ }}{I_{2 }}=\frac {{ 6 }\ \text {V}}{{ 0.8 }\ \text {A}}={ 7.5 }Ω$
$(3)R_{ 2 }'=\frac {U_{ }}{I_{ 2 }'}=\frac {{ 6 }\ \text {V}}{{ 0.3 }\ \text {A}}={ 20 }Ω$
