Divisibility: general properties · Integers

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ARMO 2020, School Round, 9.1.
ID: 52 🏷 ARMO 📅 2020 🎓 9 ★☆☆☆☆☆☆☆☆☆ Very easy
number theory divisibility four-digit numbers digit sum digit product

A four-digit number is called delightful if it is divisible by 25, the sum of its digits is divisible by 25, and the product of its digits is divisible by 25. Find all delightful numbers.

5875 and 8575

The sum of the digits of a four-digit number does not exceed 36, so for a delightful number, it must be 25. Since a delightful number is divisible by 25, it ends in either 00, 50, 25, or 75. If a four-digit number ends in 00 or 50, the sum of its digits does not exceed 9 + 9 + 5 = 23, which is not suitable. If a delightful number ends in 25, the sum of the first two digits must be 18. This could only be the number 9925, but its product of digits is not divisible by 25. Therefore, a delightful number can only end in 75. In this case, the sum of its first two digits is 13, and their product must be divisible by 5. Thus, these two digits are 5 and 8 (in any order).

ARMO 2016, School Round, 10.2.
ID: 53 🏷 ARMO 📅 2016 🎓 10 ★☆☆☆☆☆☆☆☆☆ Very easy
number theory modular arithmetic divisibility exponentiation

Is \( 13^{2013} + 13^{2014} + 13^{2015} \) divisible by 61?

Yes, it is divisible.

Transform the given sum: \( 13^{2013} + 13^{2014} + 13^{2015} = 13^{2013}(1 + 13 + 13^2) \). Notice that \( 13^2 = 169 \equiv 46 \pmod{61} \), so \( 1 + 13 + 13^2 \equiv 1 + 13 + 46 \equiv 60 \equiv -1 \pmod{61} \). Therefore, \( 13^{2013}(1 + 13 + 13^2) \equiv 13^{2013} imes (-1) \equiv -13^{2013} \equiv 0 \pmod{61} \), since \( 13^{2013} \equiv 0 \pmod{61} \). Thus, the sum is divisible by 61.

ARMO 2018, School Round, 8.4.
ID: 51 🏷 ARMO 📅 2018 🎓 8 ★★☆☆☆☆☆☆☆☆ Very easy
divisibility geometry combinatorics chess logic puzzle

Volodya placed several (possibly 0) chess pieces on an \(8 \times 8\) board. Lyonya noticed that each \(2 \times 2\) square contains the same number of pieces. Vlad noticed that each \(3 \times 1\) (or \(1 \times 3\)) rectangle contains the same number of pieces. How many pieces were placed on the board? (List all possibilities and prove there are no others.)

0 or 64

Assume each \(2 \times 2\) square contains \(m\) pieces, and each \(1 \times 3\) rectangle contains \(n\) pieces. Consider a \(2 \times 6\) rectangle on the board. On one hand, this rectangle can be divided into three \(2 \times 2\) squares, so it contains \(3m\) pieces. On the other hand, it can be divided into four \(1 \times 3\) rectangles, so it contains \(4n\) pieces. We get the equation \(3m = 4n\), implying \(n\) is divisible by 3. Since \(n\) can be 0, 1, 2, or 3, we have \(n = 0\) or \(n = 3\). In other words, either all \(1 \times 3\) rectangles are empty, resulting in 0 pieces on the board, or all \(1 \times 3\) rectangles are fully occupied, resulting in 64 pieces on the board.

ARMO 2016, School Round, 11.3.
ID: 56 🏷 ARMO 📅 2016 🎓 11 ★★☆☆☆☆☆☆☆☆ Very easy
number theory parity consecutive numbers modular arithmetic

Can the sum of 2015 consecutive natural numbers end with the same digit as the sum of the next 2019 numbers?

No, it cannot.

Method 1 (finding numbers). Let the sum of numbers from a to a + 2014 end with the same digit as the sum of numbers from a + 2015 to a + 4033. Then the difference between these sums, equal to (a + 2015 + a + 4033) - (a + a + 2014), must be divisible by 10. This simplifies to 2015 + 4033 - 2014, which equals 2019. Since 2019 is odd, it cannot be divisible by 10. Contradiction.

Method 2 (parity directly). Among 2015 + 2019 = 4034 consecutive natural numbers, there are 2017 odd numbers, which is an odd count. One sum contains an even number of odd numbers, and the other contains an odd number. Therefore, the sums will have different parity, meaning they cannot end with the same digit.

Answer. No, it cannot.

ARMO 2017, District Round, 10.2.
ID: 55 🏷 ARMO 📅 2017 🎓 10 ★★★☆☆☆☆☆☆☆ Easy
number theory divisibility integers algebra

The sum of two integers is \( S \). Masha multiplied the left number by an integer \( a \), the right one by an integer \( b \), added these products, and found that the resulting sum is divisible by \( S \).

Alyosha, on the other hand, multiplied the left number by \( b \) and the right one by \( a \).

Prove that his resulting sum is also divisible by \( S \).

Alyosha's sum is divisible by \( S \).

Let \( x \) be the left number and \( y \) be the right number; according to the condition: \( x + y = S \).

Then Masha obtained the number \( ax + by \), and Alyosha obtained the number \( bx + ay \).

The sum of these numbers is \( ax + by + bx + ay = a(x + y) + b(x + y) = (a + b)S \), which is divisible by \( S \).

Since one of the two terms (Masha's number) is divisible by \( S \), the other (Alyosha's number) must also be divisible by \( S \), as required.

ARMO 2019, District Round, 8.5.
ID: 54 🏷 ARMO 📅 2019 🎓 8 ★★★★☆☆☆☆☆☆ Easy
divisibility number theory divisors digit sum combinatorics

For a natural number \( N \), all its divisors were listed, and then the sum of the digits was calculated for each of these divisors. It turned out that among these sums, all numbers from 1 to 9 were found. Find the smallest value of \( N \).

288

Note that the number 288 has divisors 1, 2, 3, 4, 32, 6, 16, 8, 9. Therefore, this number satisfies the problem's condition. We will prove that there is no smaller number satisfying the condition. Indeed, since \( N \) must have a divisor with a digit sum of 9, \( N \) is divisible by 9. Now consider a divisor \( d \) with a digit sum of 8. \( d \) is not divisible by 3, so \( d \) and 9 are coprime, meaning \( N \) is divisible by \( 9d \). If \( d \geq 32 \), then \( 9d \geq 288 \), which implies \( N \geq 288 \). Thus, we need to check \( d = 26 \), \( d = 17 \), and \( d = 8 \). If \( d = 26 \), then \( 9d = 234 \). This number does not have a divisor with a digit sum of 5, and any number that is a multiple of it is greater than 288. If \( d = 17 \), then \( 9d = 153 \). This number does not have a divisor with a digit sum of 2, and any number that is a multiple of it is greater than 288. If \( d = 8 \), then \( 9d = 72 \). Its multiples less than 288 are 144 and 216. But these numbers do not have a divisor with a digit sum of 5.