10.9 A two-digit number divided by its reverse yields a quotient that is exactly equal to the remainder. Find this two-digit number.
(China Beijing Junior High School Grade 2 Mathematics Competition, 1991)
Problem 683
Official solution
[Solution] Let the two-digit number be , then its reverse number is , where .
From the problem, we have
That is
(1) When , (1) becomes .
This is impossible;
(2) When , (1) becomes .
Thus, is even.
When , , at this time .
When , the right side of the equation cannot be divided by 4, while the left side can be divided by 4, so there is no solution.
When , it is easy to verify that there is no solution.
Therefore, when , .
(3) When , (1) becomes
When , is not an integer.
When ,
So, when , there is no solution.
(4) When , (1) becomes
The left side of this equation can be divided by 3, but the right side cannot be divided by 3, so there is no integer solution.
(5) When , we have
If , then , thus , which is impossible.
If , then from (1) we get
This gives or , which is also impossible.
In summary, the only solution to this problem is .