volume of wire renains same so
so new resistance is 4 times that of old resistance
To do this, we the following formula
where R is resistance wire of length l, A its cross-sectional area and ρspecific resistance of material of wire, copper in the given question.
When length is doubled, volume remains same. If Anew is new area of cross section then
Remembering that ρ remains same, inserting values in (1) we get
Using (1) we get