Buy an interesting math book from 8 yuan. He has seven combinations:

1)5 yuan, 2 yuan, 1 yuan.

2)5, 1, 1, 1

3)2,2,2,2

4)2,2,2, 1, 1

5)2,2, 1, 1, 1, 1

6)2, 1, 1, 1, 1, 1, 1

7) 1, 1, 1, 1, 1, 1, 1, 1