The drift velocity of electrons in a conducting wire connected to a cell is V_d. If the length of the wire is doubled and area of cross section is halved then the drift velocity of electrons becomes

$\mathrm{We} \mathrm{know} \mathrm{i}={\mathrm{neAV}}_{\mathrm{d}}$

$\mathrm{as} \mathrm{it} \mathrm{is} \mathrm{connected} \mathrm{across} \mathrm{a} \mathrm{cell} \mathrm{V} (\mathrm{P}.\mathrm{D}) \mathrm{is} \mathrm{constant}$

$\mathrm{i}=\frac{\mathrm{V}}{\mathrm{R}}=\frac{\mathrm{VA}}{\mathrm{\rho l}}={\mathrm{neAV}}_{\mathrm{d}}$

${\mathrm{V}}_{\mathrm{d}}=\frac{\mathrm{V}}{\mathrm{ne\rho l}}$

${\mathrm{V}}_{\mathrm{d}}\propto \frac{1}{\mathrm{l}} \mathrm{but} \mathrm{independent} \mathrm{of} \mathrm{area} \mathrm{in} \mathrm{this} \mathrm{condition}$

so if l increases by two times drift velocity decreases by two times.