с - 5/9с + 4 5/6с = с * (1 - 5/9 + 4 5/6) = с * (1 - 5/9 + 29/6) = с * (54/54 - 10/54 + 261/54) = с * 305/54
При с = 7 1/5 = 36/5: (36/5) * (305/54) = (36*305)/(5*54) = 10980/270 = 1098/27 = 40.666...