Because the equation set I x+y=3 mx-ny=0 and the equation set II x-y= 1 nx+my=5 have the same solution; Then we can form a new set of equations: three x+y = three x-y= 1 and four sets of equations: mx-ny=0 nx+my=5, and three and four sets of equations are also the same solution; Solve equation group 3 to get x = 2, y =1; A new equation group 5 2m-n = 02n+m = 5； is obtained by introducing equation group 4; Solve equation group 5 and get m= 1, n=2.