$解:R_{1 }+R_2=\frac {U_{ 1}}{I_{ }}=\frac {{ 7 }\ \text {V}}{{ 0.2 }\ \text {A}}={ 35 }Ω$
$R_{ 2}+R_3=\frac {U_{2 }}{I_{ }}=\frac {{ 5 }\ \text {V}}{{ 0.2 }\ \text {A}}={ 25 }Ω$
$R_{ 1}+R_2+R_3=\frac {U_{ }}{I_{ }}=\frac {{ 10 }\ \text {V}}{{ 0.2 }\ \text {A}}={ 50 }Ω$
$聯(lián)立上式,解得R_1=25Ω,R_2=10Ω,R_3=15Ω$