 Solution

The cooling curve first falls in the entirely liquid field. On crossing the liquidus, primary Ag-rich $\alpha$ begins to grow. Latent heat release makes the descent shallower; the remaining liquid becomes richer in copper and approaches the eutectic composition.

At $779^{\circ }\mathrm{C}$, the remaining liquid solidifies as $\alpha +\beta$, giving an isothermal arrest under the ideal equilibrium, heat-extraction model. After the liquid is exhausted, temperature falls through the two-solid field. Continued loss of solid solubility causes additional redistribution and precipitation; it does not require another invariant liquid-freezing plateau.

Immediately before the eutectic reaction, the remaining liquid fraction isThe arrest represents freezing of roughly $58\%$ of the sample by mass. Its duration cannot be calculated without the heat-extraction rate and latent heat, and the onset temperature cannot be calculated exactly from the supplied phase-diagram points alone.