Maths Olympiad Prep

Track / Stage 6 / 162 of 400 #1162 of 1964

Problem 1162

National olympiad, first round
Number theory Difficulty 6.2 Find the answer

An integer consists of 7 different digits, and is a multiple of each of its digits.

What digits are in this nubmer?

A number or a short expression. Spacing and $ signs are ignored.

Official solution

1. Identify the digits that cannot be in the number:
- Digit 0: If 0 were a digit of n n , then n n would not be a multiple of 0 (since division by zero is undefined). Therefore, 0 cannot be a digit of n n .
- Digit 5: If 5 were a digit of n n , then n n must end in 5 to be divisible by 5 (since 0 is not a digit). This would make n n odd, and thus n n cannot have any even digits, which is impossible since we need 7 different digits. Therefore, 5 cannot be a digit of n n .
- Digit 4: If 4 were a digit of n n , then we need to check the sum of the digits to ensure it is a multiple of 3 and 9. Removing any one digit from {1,2,3,6,7,8,9}\{1,2,3,6,7,8,9\} and adding 4 does not result in a sum that is a multiple of 9. Therefore, 4 cannot be a digit of n n .

2. Determine the remaining digits:
- The remaining digits are {1,2,3,6,7,8,9}\{1, 2, 3, 6, 7, 8, 9\}.

3. Verify the construction:
- A valid construction of n n using these digits is n=3678192 n = 3678192 .
- Check if n n is divisible by each of its digits:
- 3678192÷1=3678192 3678192 \div 1 = 3678192 (divisible)
- 3678192÷2=1839096 3678192 \div 2 = 1839096 (divisible)
- 3678192÷3=1226064 3678192 \div 3 = 1226064 (divisible)
- 3678192÷6=613032 3678192 \div 6 = 613032 (divisible)
- 3678192÷7=525456 3678192 \div 7 = 525456 (divisible)
- 3678192÷8=459774 3678192 \div 8 = 459774 (divisible)
- 3678192÷9=408688 3678192 \div 9 = 408688 (divisible)

Since 3678192 3678192 is divisible by each of its digits, the construction is valid.

The final answer is 1,2,3,6,7,8,9\boxed{1, 2, 3, 6, 7, 8, 9}.

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