Answer
(a) decreased
(b) $-83C^{\circ}$
Work Step by Step
(a) We know that aluminum has a larger coefficient of thermal expansion as compared to that of the steel, so the temperature of the system should be increased.
(b) We know that
$\Delta T=\frac{D_{\circ}-L_{\circ}}{\alpha_{st}L_{\circ}-\alpha_{Al}D_{\circ}}$
We plug in the known values to obtain:
$\Delta T=\frac{0.100-0.0999}{(1.2\times 10^{-5})(0.0999)-(2.4\times 10^{-5})(0.100)}$
$\Delta T=-83C^{\circ}$
