Question

a wire is doubled over itself. what will be the new resistance and resistivity if the original resistance is 10 ohm?

Answer 1

If the wire is doubled then its area of crossection is double.
Resistance is inversaly propotion with area of crossection.
R=1/2 original resistance
R=5

Answer 2

When wire is doubled over itself, area is doubled and length is halved.

Resistance is proportional to Length and inversaly proportional to Area.

So

R` = R(1/2)/2

R'=R/4

R' = 10/4 =2.5 Ohm