Olympiad Maths Prep

Track / Stage 6 / 307 of 400 #1307 of 2000

Problem 1307

National olympiad, first round
Number theory Difficulty 6.6 Find the answer

For a natural number nn, let nn' denote the number obtained by deleting zero digits, if any. (For example, if n=260n = 260, n=26n' = 26, if n=2020n = 2020, n=22n' = 22.),Find the number of 33-digit numbers nn for which nn' is a divisor of nn, different from nn.

Official solution

To solve the problem, we need to consider the different cases where the number n n is a 3-digit number and n n' is a divisor of n n , but nn n' \neq n .

### Case 1: Numbers with zero digits at the end
In this case, n n can be written as n=100a+10b n = 100a + 10b where a a and b b are non-zero digits (1 to 9 for a a and 0 to 9 for b b ). The number n n' is obtained by removing the zero digit, so n=10a+b n' = 10a + b .

Since n n' is a divisor of n n , we have:
n=100a+10b n = 100a + 10b
n=10a+b n' = 10a + b

We need to check if n n' divides n n :
nn=100a+10b10a+b \frac{n}{n'} = \frac{100a + 10b}{10a + b}

For n n' to be a divisor of n n , the above fraction must be an integer. Since nn n' \neq n , we need to ensure that 10a+b100a+10b 10a + b \neq 100a + 10b .

Let's check the number of such 3-digit numbers:
- a a can be any digit from 1 to 9 (9 choices).
- b b can be any digit from 0 to 9 (10 choices).

Thus, there are:
9×10=90 9 \times 10 = 90
such numbers.

### Case 2: Numbers with one zero in the middle
In this case, n n can be written as n=100a+100+b=100a+b n = 100a + 10 \cdot 0 + b = 100a + b where a a and b b are non-zero digits (1 to 9 for a a and 1 to 9 for b b ). The number n n' is obtained by removing the zero digit, so n=10a+b n' = 10a + b .

We need to check if n n' divides n n :
n=100a+b n = 100a + b
n=10a+b n' = 10a + b

We need to check if n n' divides n n :
nn=100a+b10a+b \frac{n}{n'} = \frac{100a + b}{10a + b}

For n n' to be a divisor of n n , the above fraction must be an integer. Simplifying the fraction:
100a+b10a+b=10+90a10a+b \frac{100a + b}{10a + b} = 10 + \frac{90a}{10a + b}

For this to be an integer, 90a10a+b \frac{90a}{10a + b} must be an integer. We can check the possible values of a a and b b that satisfy this condition.

By checking all values of a a from 1 to 9 and b b from 1 to 9, we find the following pairs:
(a,b)=(1,5),(1,8),(4,5) (a, b) = (1, 5), (1, 8), (4, 5)

Thus, there are 3 such numbers.

### Conclusion
Adding the numbers from both cases, we get:
90+3=93 90 + 3 = 93

The final answer is 93\boxed{93}.

Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic, difficulty and ordering added by this site.