d

2

d

three

c

four

b

five

d

six

a

seven

d

eight

d

nine

4/ 1

10

The answer is not unique.

1 1

The answer is not unique.

12

Addition, subtraction and elimination

Substitution elimination

13

three

-2

14

{ x = 10y = 1z =- 16

15

-4

16

Win 1, draw 3 or win 2, lose 2.

17

( 1)x=2/ 1

y=2/3

(2)

x= 17

y=8

18

x=- 1

y=-2

z= 1

19

x=370

y= 1 10

20

m=0，3，4，5

2 1

6 in 20 yuan and 20 in 5 yuan.