x＋y=m (1) 5x+3y=31(2)(1)×3得 3x+3y=3m (3)(2)-(3)得 2x=31-3mx=(31-3m)/2代入（1）得y=(5m-31)/2∵解为非负数∴﹛(31-3m)/2≥0 m≤31/3 (5m-31)/2≥0 m≥31/5∴31/5≤m≤31/3∵m是整数∴m=7, 8, 9, 10
