d
For \(ADE\) \(\frac{1}{\mathrm{R}^{\prime}}=\frac{1}{2 \mathrm{x}}+\frac{1}{10}\)

or \(\quad \mathrm{R}^{\prime}=\frac{20 \mathrm{x}}{10+2 \mathrm{x}}\)

\(\mathrm{R}_{\mathrm{BC}}=\frac{20 \mathrm{x}}{10+2 \mathrm{x}}+20-\mathrm{x}+20-\mathrm{x}\)        ....\((i)\)

or \(\frac{20 x}{10+2 x}+40=2 x\)

Solving we get

\(x=10\, \Omega\)

Putting the value of \(x=10\, \Omega\) in equation \(( i )\) We get

\( R_{B C} =\frac{20 \times 10}{10+2 \times 10}+20-10+20-10 \)

\(=\frac{80}{3}=26.7\, \Omega \)