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Difficulty: 5/102024 IOQM 2024 (Q21)

An integer is such that is a three-digit number with equal digits, and is a four-digit number with the digits in some order. What is the remainder when is divided by ?

Guide / Hint

Hint 1: Let and . List the possible values of (permutations of ) and (three-digit identical digit numbers like ).

Hint 2: Write down the inequalities for based on the floor definitions: and .

Hint 3: Show that only and provide overlapping intervals for . Determine and calculate its remainder modulo 100.

Solution

Step 1: Let . Since is a four-digit number composed of the digits in some order, the possible values for are and .

Step 2: From , we have:

Step 3: We are also given that is a three-digit number with equal digits. Thus, . This implies:

Step 4: Combining the two inequalities for :

which simplifies to:

Step 5: We test the possible values of to find one that yields one of the valid values for . Let us try :

Since must be one of the permitted permutations of , we see that lies perfectly in this range.

Step 6: With , the first inequality gives:

And with , the second inequality gives:

Step 7: The only integer satisfying both conditions is .

Step 8: Finally, we find the remainder when is divided by :

Thus, the remainder is .

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