derive the lever rule for a general mixture of two phases, a and b. let the composition of the alloy be c (wt% of alloying element), the compositions of the phases be ca and cb, and the weight fractions of the phases be wa and wb . [hints: first find an expression conserving the mass of the alloying element between the alloy and the two phases, then define a, b and ‘ in this notation and use the overall conservation of mass expressed in wa þ wb ¼ 1.]