Hello Kusm,

I would try to explain you with a help of an example below.

Let the length of wire = 100 cm.

Let its area of cross section = a cm²

Then resistance R of wire= resistivity× 100/a….(1)

Length of wire after stretching = 110 cm

Let the new area of cross section = a' cm²

Volume of wire must be same before and after stretching, ie

100 a = 110 a' or a' = 10a/ 11 cm²

New resistance R'= (resistivity)×110× 11/10a= (resistivity)× 121/a …………….(2)

Dividing (2) by (1) we get,

R'/R = 121/100

( R' - R/R) = 121–100/100= 21/100

Therefore % change= 21% increase in resistance.

So I hope this solves your doubt and if you have any more doubt then you can comment down your query in the comments section.

All the best to you!