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Difficulty: 4/102024 IOQM 2024 (Q8)

Let n, be the smallest integer such that the sum of digits of is divisible by 5 as sell as the sum of digits of (0 + 1) is divisible by 5. What are the first two digits of in the same order?

Guide / Hint

Hint 1: Start by analyzing the initial conditions and setting up the basic equations. Let sum of digits of n-digits number = 5.

Hint 2: Look for algebraic properties, symmetry, or geometric theorems to simplify. Let sum of digits of + 1 number = 5k.

Hint 3: Proceed with the final algebraic steps to solve the system. \dots, 999, 1000, \dots, 9999, 10,000.

Solution

Step 1: Let sum of digits of n-digits number = 5

Step 2: Let sum of digits of + 1 number = 5k

Step 3: \dots, 999, 1000, \dots, 9999, 10,000

Step 4: 26 35

Step 5: \dots, 49999, 50000

Step 6: => Smallest number = 49,999

Step 7: Firsts two digits = 49.

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