Determine the composition of each phase in a cu-40% ni alloy at 1300c,

To be able to answer this, you need to have a phase diagram of Cu-Ni alloy. Create a vertical line along 40% in the x-axis. Then, create a horizontal line along the temperature at 1,300°C. The intersection lies outside of the α-L boundary. Therefore, the system consist solely of the L-phase: 100% L.