Ký hiệu abcd là số tự nhiên có 4 chữ số. 
abcd + abc + ab + a = 1111.a + 111.b + 11.c + d 
Vậy 1111.a + 111.b + 11.c + d = 4321 
+ Nếu a < 3 => 111.b + 11.c + d > 2098 (không được vì b, c, d < 10) 
+ Nếu a > 3 => vế trái > 4321 
Vậy a = 3 => 111.b + 11.c + d = 988 
+ Nếu b < 8 => 11.c + d > 210 (không được vì c, d < 10) 
+ Nếu b > 8 => vế trái > 988 
Vậy b = 8 => 11.c + d = 100 
+ Nếu c < 9 => d > 11 (không được) 
Vậy c = 9; d = 1 
=> (abcd) = 3891

d) => 5

a) 4x + 18 : 2 = 13

4x + 9 = 13

4x = 13 - 9

4x = 4

x = 4:4

x = 1

b) 130 - (100 + x) = 25

100 + x = 130 - 25

100 + x = 105

x = 105 - 100

x = 5

c) 5 (x + 12) + 22 = 92

5 (x + 12) = 92 - 22

5 (x + 12) = 70

x + 12 = 70 : 5

x + 12 = 14

x = 14 - 12

x = 2

d) 260 - 10 ( 24 - 3x ) : 15 = 244

250 - 10 (24 - 3x) : 15 = 244

10(24 - 3x) : 15 = 250 - 244

10 (24 - 3x) : 15 = 6

10(24 - 3x) = 6 x 15

10(24 - 3x) = 90

24 - 3x = 90 : 10

24 - 3x = 9

3x = 24 - 9

3x = 15

x = 15 : 3

x = 5

5, " When does the last train leave?

⇒ I asked when the last train left ( S+ asked +wh-question +S+V)

6," How long did your brother work here?

⇒ She asked him how long his brother worked there(S+asked +wh-question+S+V )

1,What grade are Nhung in? 

(dịch: Nhung học lớp mấy ? )
2,Where are the student ?     

(dịch: Học sinh đang ở đâu? )

Lan asked Ha what she would do if she were having a problem.

=> Câu gián tiếp Wh-questions:

 S + asked sb + wh-word + clause(lùi thì)

Câu điều kiện loại 2 trong câu tường thuật không cần lùi thì.

7.Will she be on the beach next week ?

8.i think i and my brother will be at school   

9.we will have seafood and noodles for dinner 

25. previous day

24.Jim asked me what I often did if I was sad

25.Jim asked me who I had run into the previous day

 He told me that he was sure she would help me if i asked her