Maths Olympiad Prep

Track / Stage 6 / 332 of 400 #1332 of 1964

Problem 1332

National olympiad, first round
Combinatorics Difficulty 6.7 Find the answer

Thirty numbers are arranged on a circle in such a way that each number equals the absolute difference of its two neighbors. Given that the sum of the numbers is 20002000, determine the numbers.

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Official solution

1. Let's denote the 30 numbers as a1,a2,,a30a_1, a_2, \ldots, a_{30} arranged in a circle. This means a31=a1a_{31} = a_1.
2. According to the problem, each number equals the absolute difference of its two neighbors. Therefore, we have:
ai=ai1ai+1 a_i = |a_{i-1} - a_{i+1}|
for i=1,2,,30i = 1, 2, \ldots, 30.
3. Given that the sum of the numbers is 2000:
i=130ai=2000 \sum_{i=1}^{30} a_i = 2000
4. Let's consider the solution provided: the numbers are composed of 10 blocks of three numbers, and each block has the numbers 100,0,100100, 0, 100 in that order.
5. We can verify this solution by checking if it satisfies the conditions:
- Each block is 100,0,100100, 0, 100.
- The absolute difference condition:
1000=100,0100=100,100100=0 |100 - 0| = 100, \quad |0 - 100| = 100, \quad |100 - 100| = 0
This satisfies the condition for each block.
6. Since there are 10 such blocks, the total sum is:
10×(100+0+100)=10×200=2000 10 \times (100 + 0 + 100) = 10 \times 200 = 2000
This matches the given sum of 2000.
7. Therefore, the numbers can indeed be arranged in 10 blocks of 100,0,100100, 0, 100.

The final answer is 100,0,100 \boxed{ 100, 0, 100 } repeated 10 times.

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