abcd = 1000 x a,bcd
abc,d = 100 x a,bcd
ab.cd = 10 x a,bcd
Nên:
abcd + abc,d + ab,cd + a,bcd = a,bcd x (1000 + 100 + 10 + 1) = 1111 x a,bcd
Hay 
1111 x a,bcd = 2238,665
a,bcd = 2238,665 : 1111
a,bcd = 2,015
Vậy:
abcd = 2015

các số có 3 chữ số là 102;...........;996

ta có tất cả 150  chia hết cho 2 và 3 

vì (996-102) :6 = 150 số

(5x +335) : x = 72

5x + 335 = 72x

67x = 335

x = 5

đổ từ thùng 7lit

theo bài gọi số trẻ là a thì số con ngựa là a-1 (1)

mặt khác hai đứa trẻ cưỡi một con thì thừa một con ngựa vậy:

a /2 = (a-1) -1

=> a = 2*(a - 2)

=> số trẻ là : a = 4 (đứa trẻ)

theo (1) số ngựa là: a-1 = 4-1 = 3 (con ngựa)

ĐS:

1. the school authorities can't set holiday based on local weather conditions .

➜I wish the school authorities could set holiday based on local weather conditions .

2. that university often often doesn't attract enough environments for the new school year .

➜I wish that university often attracted enough environments for the new school year .

3. we can't study the books written by famous scholars in the Ly and Tran dynasties because Chinese Ming aggressors took thêm to China .

➜ I wish we could study the books written by famous scholars in the Ly and Tran dynasties because Chinese Ming aggressors took thêm to China .

4 . many small children don't learn to swim so they often suffer from drowning

➜ I wish many small children learned to swim so they often suffer from drowning

5. many gradutes from college don't have enough skills to meet the requirements of their jobs

➜I wish many gradutes from college had enough skills to meet the requirements of their jobs