Đáp án + Giải thích các bước giải:

`1)1111 = 1 xx 1000 + 1 xx 100 + 1 xx 10 + 1`

`= 1 xx 10^3 + 1 xx 10^2 + 1 xx 10^1 + 1`

`2)1247 = 1 xx 1000 + 2 xx 100 + 4 xx 10 + 7`

`= 1 xx 10^3 + 2 xx 10^2 + 4 xx 10^1 + 7`

`3) 1543 = 1 xx 1000 + 5 xx 100 + 4 xx 10 + 3`

`= 1 xx 10^3 + 5 x 10^2 + 4 x 10^1 + 3`

`4) 1794 = 1 xx 1000 + 7 xx 100 + 9 xx 10 + 4`

`= 1 xx 10^3 + 7 xx 10^2 + 9 xx 10^1 + 4`

`5) 1876 = 1 xx 1000 + 8 xx 100 + 7 xx 10 + 6`

`= 1 xx 10^3 + 8 xx 10^2 + 7 xx 10^1 + 6`

`6) 1008 = 1 xx 1000 + 8`

`= 1 xx 10^3 + 8`

`7) 1080 = 1 xx 1000 + 8 xx 10`

`= 1 xx 10^3 + 8 xx 10^1`

`8) 1709 = 1 xx 1000 + 7 xx 100 + 9`

`= 1 xx 10^3 + 7 xx 10^2 + 9`

`9) 1500 = 1 xx 1000 + 5 xx 100`

`= 1 xx 10^3 + 5 xx 10^2`

`10) 1097 = 1 xx 1000 + 9 xx 10 + 7`

`= 1 xx 10^3 + 9 xx 10^1 + 7`

`11) 20000 = 2 xx 10000`

`= 2 xx 10^4`

`12) 27000 = 2 xx 10000 + 7 xx 1000`

`= 2 xx10^4 + 7 xx 10^3`

`13) 28700 = 2 xx 10000 + 8 xx 1000 + 7 xx 100`

`= 2 xx 10^4+ 8 xx 10^3 + 7 xx 10^2`

`14) 24004 = 2 xx 10000 + 4 xx 1000 + 4`

`= 2 xx10^4 + 4 xx 10^3 + 4`

`15) 26013 = 2 xx 10000 + 6 xx 1000 + 1 xx 10 + 3`

`= 2 xx 10^4 + 6 xx 10^3 + 1 xx 10^1 + 3`

`16) 20508 = 2 xx 10000 + 5 xx 100 + 8`

`= 2 xx10^4 + 5 xx 10^2 + 8`

`17) 20358 = 2 xx 10000 + 3 xx 100 + 5 xx 10 + 8`

`= 2 xx 10^4 + 3 xx 10^2 + 5 xx 10^1 + 8`

`18) 20009 = 2 xx 10000 + 9 = 2 xx 10^4 + 9`

`19) 29830 = 2 xx 10000 + 9 xx 1000 + 8 xx 100 + 3 xx 10`

`= 2 xx 10^4 + 9 xx 10^3 + 8 xx 10^2 + 3 xx 10^1`

`20) 25763 = 2 xx 10000 + 5 xx 1000 + 7 xx 100 + 6 xx 10 + 3`

`= 2 xx10^4 + 5 xx 10^3 + 7 xx 10^2 + 6 xx 10^1 + 3`

`text{#Study Well}`

1, 1111 = 1 x 1000 + 1 x 100 + 1 x 10 + 1 = 1 x 10³ + 1 x 10² + 1 x $10^{1}$ + 1

2, 1247 = 1 x 1000 + 2 x 100 + 4 x 10 + 7 = 1 x 10³ + 2 x 10² + 4 x $10^{1}$ + 7

3, 1543 = 1 x 1000 + 5 x 100 + 4 x 10 + 3 = 1 x 10³ + 5 x 10² + 4 x $10^{1}$ + 3

4, 1794 = 1 x 1000 + 7 x 100 + 9 x 10 + 4 = 1 x 10³ + 7 x 10² + 9 x $10^{1}$ + 4

5, 1876 = 1 x 1000 + 8 x 100 + 7 x 10 + 6 = 1 x 10³ + 8 x 10² + 7 x $10^{1}$ + 6

6, 1008 = 1 x 1000 + 8 = 1 x 10³ + 8

7, 1080 = 1 x 1000 + 8 x 10 = 1 x 10³ + 8 x $10^{1}$ 

8, 1709 = 1 x 1000 + 7 x 100 + 9 = 1 x 10³ + 7 x 10² + 9

9, 1500 = 1 x 1000 + 5 x 100 = 1 x 10³ + 5 x 10²

10, 1097 = 1 x 1000 + 9 x 10 + 7 = 1 x 10³ + 9 x $10^{1}$ + 7

11, 20000 = 2 x 10000 = 2 x $10^{4}$ 

12, 27000 = 2 x 10000 + 7 x 1000 = 2 x $10^{4}$ + 7 x 10³ 

13, 28700 = 2 x 10000 + 8 x 1000 + 7 x 100 = 2 x $10^{4}$ + 8 x 10³ + 7 x 10² 

14, 24004 = 2 x 10000 + 4 x 1000 + 4 = 2 x $10^{4}$ + 4 x 10³ + 4

15, 26013 = 2 x 10000 + 6 x 1000 + 1 x 10 + 3 = 2 x $10^{4}$ + 6 x 10³ + 1 x $10^{1}$ + 3

16, 20508 = 2 x 10000 + 5 x 100 + 8 = 2 x $10^{4}$ + 5 x 10² + 8

17, 20358 = 2 x 10000 + 3 x 100 + 5 x 10 + 8 = 2 x $10^{4}$ + 3 x 10² + 5 x $10^{1}$ + 8

18, 20009 = 2 x 10000 + 9 = 2 x $10^{4}$ + 9

19, 29830 = 2 x 10000 + 9 x 1000 + 8 x 100 + 3 x 10 = 2 x $10^{4}$ + 9 x 10³ + 8 x 10² + 3 x $10^{1}$ 

20, 25763 = 2 x 10000 + 5 x 1000 + 7 x 100 + 6 x 10 + 3 = 2 x $10^{4}$ + 5 x 10³ + 7 x 10² + 6 x $10^{1}$ + 3