A NiO$-20$ mol$\%$ MgO ceramic is allowed to solidify. $($See the figure below.$)$

$($a$)$ Determine the composition $($in mol$\%$MgO $)$ of the first solid to form.

$\%$

$($b$)$ Determine the composition $($in mol$\%$MgO $)$ of the last liquid to solidify under equilibrium conditions.

$\%$

im confused