There is a four digit number in the first day of math problem, which satisfies the following conditions: (1) the sum of two times of the number in one digit and two is less than half of the number in ten digit. (2) the number in ten digit There is a four digit number, which satisfies the following conditions: (1) the sum of two times and two is less than half of the ten digit number. (2) the number of one digit is transferred with the number of thousand digit, the number of ten digit is transferred with the number of hundred digit, and the new four digit number is the same as the original four digit number. (3) the sum of two digits and ten digit number is 10

There is a four digit number in the first day of math problem, which satisfies the following conditions: (1) the sum of two times of the number in one digit and two is less than half of the number in ten digit. (2) the number in ten digit There is a four digit number, which satisfies the following conditions: (1) the sum of two times and two is less than half of the ten digit number. (2) the number of one digit is transferred with the number of thousand digit, the number of ten digit is transferred with the number of hundred digit, and the new four digit number is the same as the original four digit number. (3) the sum of two digits and ten digit number is 10

The sum of one digit and ten digit is 10, and the sum of two times and two is less than half of ten digit. By reasoning, it can be concluded that one digit is 1 and nine digit is 9
The new four digits are the same as the original four digits, that is, the one digit is the same as the thousand digit, and the ten digit is the same as the hundred digit
So this four digit number is 1991