= Solution
The room-temperature endpoints are pure Ag and pure Cu. Conservation of the total <copper> <mass> therefore gives
$$
\boxed{w_\alpha=1-0.28=0.72,\qquad w_\beta=0.28.}
$$
The answer is \b[72% <silver> phase and 28% <copper> phase by <mass>]. The change from the high-temperature fractions is caused by decreasing solid solubility: the phases reject the opposite component during equilibrium cooling. Simply retaining the eutectic-temperature lever-rule fractions would violate the stipulated room-temperature equilibrium compositions.
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