Question

Follow-Up: Now let’s define an interesting number as a 10-digit number divisible by 11,111 with DISTINCT digits. How many interesting numbers are there?

Answer 1

To find the number of interesting numbers, we need to find the number of 10-digit numbers that are divisible by 11,111 and have distinct digits.

A number is divisible by 11,111 if and only if the alternating sum of its digits is divisible by 11. For example, 27,183,642 is divisible by 11,111 because 2 - 7 + 1 - 8 + 3 - 6 + 4 - 2 = -3, which is divisible by 11.

Since we want the number to have distinct digits, we can choose any 5 digits from the set {0, 1, 2, 3, 4, 5, 6, 7, 8, 9} and arrange them in any order. There are 10 choices for the first digit, 9 choices for the second digit (since we can't repeat the first digit), 8 choices for the third digit, and so on, giving us a total of 10 x 9 x 8 x 7 x 6 = 30,240 possible 5-digit numbers with distinct digits.

To form a 10-digit number, we can repeat the 5 digits in any order. There are 5! = 120 ways to arrange the 5 digits, so there are 120 different 10-digit numbers that can be formed from each 5-digit number.

Therefore, the total number of interesting numbers is:

30,240 x 120 = 3,628,800

So there are 3,628,800 interesting numbers that are 10-digit numbers divisible by 11,111 with distinct digits.

Answer 2

5,040

Explanation:

There are a total of 5,040 interesting numbers, which can be calculated using basic combinatorics. To calculate this, the first step is to count the total number of 10-digit numbers with distinct digits. Since there are 10 distinct digits (0, 1, 2, 3, 4, 5, 6, 7, 8, and 9), there are 10! (10 x 9 x 8 x 7 x 6 x 5 x 4 x 3 x 2 x 1) possible numbers with distinct digits. The next step is to count the number of 10-digit numbers divisible by 11,111. To do this, we need to find the number of 10-digit numbers divisible by 11. Since 11 is a prime number, there are 11 possible combinations of 10-digit numbers divisible by 11. For example, 0XXXXXXXXX, 1XXXXXXXXX, 2XXXXXXXXX, etc. So, there are 11 x 10! (110,000) possible 10-digit numbers divisible by 11. Finally, to find the number of interesting numbers, we need to subtract the number of 10-digit numbers divisible by 11,111 from the total number of 10-digit numbers with distinct digits. This gives us 10! - 11 x 10! = 5,040. Therefore, there are a total of 5,040 interesting numbers.