Let the original length = L₁
Let the original resistance = R₁ = 5Ω
Let the original resistivity = P₁
Let the original area = a₁
Let the new length = L₂ = 2L₁
let the new area = a₂ = 1/(2a₂)
Let the new resistance = R₂
Let the new resistivity = P₂
But since the resistivity remains the same,
=> P₁ = P₂∴ P₁ = R₁ a₁

_L1

= P₂ = R₂ a₂

_L2

But a₂ = ¹/₂a₁; and L₂ = 2L₁∴ R₁ a₁

_L1

= R₂ ^2a₁/₂

_2L1

=> 5 x a₁

_L1

= R₂ x a₁

_4L1

∴R₂ = 5 x a₁ x 4L₁

_a1 x L1

= 20Ω