Deformation of a dicomponent rod under tension

1. The problem statement, all variables and given/known data



2. Relevant equations



3. The attempt at a solution
For this problem, my thinking was that since the top of the rod is fixed, then the only part that is elongating is the bottom, so I found the part elongating at AB and said that was the total elongation. I am not sure if this reasoning is sound. I suppose the other possibility would be to add up AB and BC to get the total, but then I don't know what the deformation at point B means. I am interpreting that as the elongation of BC.

 

Wouldn't that mean that you need the answer to part b to solve part a? That is why I am confused I think

 

Here is my attempt 2. How does it look?