Dear student,
We know that,
V=IR
and,
R=ρL/A
Since the two wires of the same material are connected in series, the current flowing through them will be same and hence potential difference across the wire will be directly proportional to L/A.
V=IR=IρL/A=IρL/(d/2)2
V∝l /d2
So, the ratio of potential difference will be
=9/8
Thus, the ratio of potential difference is 9:8.
Regards