$\ell^{\prime} A =\ell A$

$2 \ell A ^{\prime}=\ell A$

$A ^{\prime}=\frac{ A }{2}$

$R =\rho \frac{\ell}{ A } \ldots\,\, (i)$

$\ell^{\prime}=2 \ell$

$A ^{\prime}=\frac{ A }{2}$

$R ^{\prime}=\frac{\rho \ell^{\prime}}{ A }=\frac{\rho 2 \ell}{\frac{ A }{2}}$

$R ^{\prime}=\frac{4 \rho \ell}{ A }$

$R ^{\prime}=4 R$ from equation $(i)$

$\%$ increase in resistance

$=\frac{ R ^{\prime}- R }{ R } \times 100=\frac{4 R - R }{ R } \times 100$

$=300 \%$