I think it equals 4 or 6 or 8
Answer:

Step-by-step explanation:
We know (Ohm second law) that
where k inlcudes the rest of the parameters (material, lenght). In our situation we have k=
. The moment the diameter becomes 8mm R becomes

A + b = 15 ..... eq 1
a – b = 12 ...... eq 2
So a = 15 – b
Substitute in eq 2 :
15 – b – b = 12
15 – 2b = 12
–2b = 12 – 15
b = 1.5
So a = 15 – 3/2 = 13.5
4ab = 4 x 13.5 x 1.5 = 81