Solve a super difficult math problem!! Fast!!

First of all, sorry, the system may change my plus sign to a space, so where there is an inexplicable space is the plus sign! Here's the process. Because CDBA is a multiple of 9, a b c d is a multiple of 9, so both abcd and bcda are multiples of 9.

And ABCD BCDA=1001*A 11*BCD=11*91*A 11*BCD is a multiple of 11, and BCDA is also a multiple of 11. So ABCD is a multiple of 11. Therefore, ABCD must be a multiple of 9*11*13=1287.

After verification, you will find that most of these multiples are answers! 1287;2574;3861;5148;6435

Hello! There are 5 answers that all meet the conditions, which are: 1, 2, 8, 7

Analyze, multiply 13 by 11 by 9 to get the number and multiply it by the numbers 1, 2, 3, 4, 5.

CDAB is a multiple of 9, so a+b+c+d is a multiple of 9.

So any combination of ab, c, and d is a multiple of 9.

10*abcd=10*(1000*a+bcd)=10000*a+10*bcd

10*bcd+a)+9999*a

bcda+9999*a

Because bcda is a multiple of 11, 9999*A is also a multiple of 11.

So 10*abcd is a multiple of 11.

So ABCD is a multiple of 9, 11, 13, (1287), and a multiple of 1287 is a quadruple of four digits without repeating numbers: 1287 2574 3861 5148 6435

So the four numbers of ABCD are:

1 2 8 7 or 2 5 7 4 or 3 8 6 1 or 5 1 4 8 or 6 4 3 5

10 pcs. The first person sends his answer to the second person, and the second person, after receiving it, sends the first person's answer to the third person along with his own answer, and so on, and the ninth person sends his answer to the tenth person along with the answers of the first eight people. Then the tenth person came to ** and sent the answers of the ten people to the other nine people together.

After using the average of 2,000 numbers on the calculator, Ma Xiaoha accidentally mixed the average with the original 2,000 numbers. Interestingly, the average of these 2001 numbers happens to be 2001. Originally, the average of these 2,000 numbers was 2001

Because the sum of the average and the original 2000 numbers is the 2001 average, the average is 2001