Question

Wire of resistivity is stretched to twice its original length without changing number density what will be

Answer 1

As we know that ;
Resistance = Resistivity*length/area
Here we note that the resistance is directly proportional to length and inversely proportional to area of cross section .... 
Now a/c to question,;
Length of wire is double then area is reduced to half...
Now 
New resistance = resistivity*2length/area/2
New resistance =(resistivity*length/area)*4
New resistance = 4resistance