Answer
$1.6\times10^{-3}cm$.
Work Step by Step
As stated in the problem, the gap is the inside radius of the lid minus the outside radius of the jar. Assume that the original radii of the lid and of the jar are equal to 4.0 cm.
$$gap=(r_o+\Delta r)_{lid}-(r_o+\Delta r)_{jar}=\Delta r_{lid}-\Delta r_{jar}=(\alpha_{brass}-\alpha_{glass})r_o \Delta T $$
$$=(19\times10^{-6}/C^{\circ}-9\times10^{-6}/C^{\circ})(4.0cm)(40C^{\circ})=1.6\times10^{-3}cm $$
