The formula to calculate the Young's modulus of the material of the wire, in the first case, can be written as
Y=Llπr2F...(1)
From equation (1), it follows that
F=Yπr2×Ll...(2)
For the second case, the Young's modulus is given by
Y=Ll′πr2/4F/2...(3)
From equation (3), it follows that
F=YLl′×2×4πr2...(4)
From equations (2) and (4), it implies that
Yπr2×Ll=YLl′×2×4πr2⇒l′=2l