= Solution
At $125\,{}^\circ\mathrm C$, primary Bi-rich crystals are surrounded by a transformed two-phase <eutectic microstructure>. Reading approximately $x_B=0.026$ and $x_S=0.94$ gives phase compositions $1.49$ and $89.90\,\mathrm{wt\%}$ Sn. Consequently,
$$
\boxed{f_{\beta\rm Sn}\simeq\frac{0.1958-0.0149}{0.8990-0.0149}=0.205,\qquad f_{\rm Bi}\simeq0.795.}
$$
These are total \b[phase mass fractions], including both primary and eutectic material. They differ from the <microconstituent> fractions: just before the eutectic reaction, roughly $39\%$ by mass is <liquid> and subsequently becomes the eutectic constituent, with roughly $61\%$ primary Bi-rich material. Further slow cooling changes the phase compositions and allows secondary precipitation. Faster cooling usually refines the primary crystals and eutectic spacing and increases retained segregation; the equilibrium diagram does not specify the resulting nonequilibrium fractions uniquely.
\Image[/past-exam-of-the-materials-science-course-of-the-university-of-cambridge/2019/paper-1-eutectic-microstructure.png]
{title=Primary crystals with liquid and with a solidified eutectic constituent}
{description=Original microstructure cartoons. Areas illustrate the distinction between primary material and eutectic constituent; they are not calibrated mass or volume fractions.}
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