APb$-Sn$ alloy containing $20wt\%Sn$ is heated from room temperature to $100C.$

$($a$)$ What phases are present at $100C?(0\mathrm{.}5$ point$)$

$($b$)$ Give the compositions of the phases at $100C.(0\mathrm{.}5$ point$)$

The same $Pb-20wt\%Sn$ alloy in question $1$ is now heated to $185C.$

$($a$)$ What phases are present at $185C?(0\mathrm{.}5$ point$)$

$($b$)$ What are the phase fractions at $185C?(0\mathrm{.}5$ point$)$