Hd: $\begin{align}& {{R}_{1}}={{R}_{2}}+5 \\& \frac{U}{R}=\frac{{{U}_{1}}}{{{R}_{1}}}=\frac{{{U}_{2}}}{{{R}_{2}}} \\& \Rightarrow \frac{30}{5+2{{R}_{2}}}=\frac{30}{5+{{R}_{2}}}=\frac{20}{{{R}_{2}}} \\& \Rightarrow 30{{R}_{2}}=100+20{{R}_{2}} \\& \Rightarrow 10{{R}_{2}}=100 \\& \Rightarrow {{R}_{2}}=10\Omega  \\& \Rightarrow {{R}_{1}}=10+5=15\Omega  \\\end{align}$

CHỌN ĐÁP ÁN B