Apply the lever rule on the

77 9^{\circ} C

solid tie line. All compositions are copper mass percentages:

w_{α} = \frac{92 - 28}{92 - 9} = \frac{64}{83} = 0.7711, w_{β} = \frac{28 - 9}{92 - 9} = \frac{19}{83} = 0.2289.

(1)

Thus the equilibrium material is 77.1% $α$ and 22.9% $β$ by mass just below the eutectic temperature. These are phase fractions; converting them to volume fractions would require densities.

The answer is 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.