Finally, \(\frac{ P + x }{ Q }=\frac{80 cm }{20 cm }=\frac{4}{1} \ldots(2)\)
Divide \((2)\) by \((1)\)
\(\frac{ P + x }{ P }=4 \times \frac{3}{2}=6\)
\(1+\frac{ x }{ P }=6 \Rightarrow \frac{ x }{ P }=5\)
\(\therefore x =5 P =5 \times 4=20 \Omega\)