For a natural number , let denote the number obtained by deleting zero digits, if any. (For example, if , , if , .),Find the number of -digit numbers for which is a divisor of , different from .
Problem 1307
Official solution
To solve the problem, we need to consider the different cases where the number is a 3-digit number and is a divisor of , but .
### Case 1: Numbers with zero digits at the end
In this case, can be written as where and are non-zero digits (1 to 9 for and 0 to 9 for ). The number is obtained by removing the zero digit, so .
Since is a divisor of , we have:
We need to check if divides :
For to be a divisor of , the above fraction must be an integer. Since , we need to ensure that .
Let's check the number of such 3-digit numbers:
- can be any digit from 1 to 9 (9 choices).
- can be any digit from 0 to 9 (10 choices).
Thus, there are:
such numbers.
### Case 2: Numbers with one zero in the middle
In this case, can be written as where and are non-zero digits (1 to 9 for and 1 to 9 for ). The number is obtained by removing the zero digit, so .
We need to check if divides :
We need to check if divides :
For to be a divisor of , the above fraction must be an integer. Simplifying the fraction:
For this to be an integer, must be an integer. We can check the possible values of and that satisfy this condition.
By checking all values of from 1 to 9 and from 1 to 9, we find the following pairs:
Thus, there are 3 such numbers.
### Conclusion
Adding the numbers from both cases, we get:
The final answer is .