A brass ring of diameter 10.00 cm at 19.9°C is heated and slipped over an aluminum rod of diameter 10.01 cm at 19.9°C. Assume the average coefficients of linear expansion are constant.

(a) To what temperature must the combination be cooled to separate the two metals?

(c) What if the aluminum rod were 10.96 cm in diameter?

i used the equation r + alpha(deltaT) = r +ralpha(deltaT)

and then i solved for T and i ended up getting 19.9 degrees C...which is what I started with

I don't know what I am supposed to do I guess. :|