A $50w1\%Pb-50w1\%Mg$ alloy is slowly cooled from $700C(973K)$ to $400C(673K).$

$($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?