1.Many—inventor—is ancient ,China—invented—a lot of important—invention—.
2.China is a—developing—country,while the USA is a—developed—one
3.Abacus is a —calculating—machine to do —calculations—,that is,abacus is a kind of—calculator—.
4.The superman is a very—powerful—man in our imagination.(power)
5.15—subtracted—from 20 is 5.—subtract—15 from 20,and you have 5.—subtract—15 from 20,and you have 5(subtract)
6.20—minus—15 is 5.(minus) —Thousands—of people are spending their holidays in this little city.(thousand)