$9\mathrm{.}23\mathrm{.}14.$ A $50wt\%Pb-50wt\%Mg$ alloy is slowly cooled from $700C(1290F)$ to $400C(750F).$

$($a$)$ At what temperature does the first solid phase form?

$($b$)$ What is the composition of this solid phase?

$($c$)$ At what temperature does the liquid solidify?

$($d$)$ What is the composition of this last remaining liquid phase?