当他们的单位不一样的情况下.
1只羊 + 1只羊 = 1群羊;
2只羊 + 1只羊 = 1群羊;
3只羊 + 4只羊 = 1群羊;
4只羊 + 9只羊 = 1群羊;
5只羊 + 7只羊 = 1群羊;
6只羊 + 18只羊 = 1群羊;