Difference between revisions of "2002 OIM Problems/Problem 1"

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== Problem ==
 
== Problem ==
a) There are two sequences, each of 2003 consecutive integers, and a board with 2 rows and 2003 columns
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The integers from 1 to 2002, both inclusive, are written on a blackboard in increasing order <math>1, 2, \cdots , 2001, 2002</math>. Then, those in first, fourth place, seventh place, etc. are erased, that is, those who occupy the places of the form <math>3k + 1</math>. In the new list the numbers that are in the places of the form <math>3k + 1</math> are erased. This process is repeated until all numbers are deleted from the list. What was the last number that was erased?
 
Decide if it is always possible to distribute the numbers of the first sequence in the first row and those of the second sequence in the second row, in such a way that the results obtained by adding the two numbers in each column form a new sequence of 2003 consecutive numbers.
 
 
 
b) What if 2003 is replaced by 2004?
 
 
 
In both a) and b), if the answer is affirmative, explain how you would distribute the numbers, and if the answer is negative, justify why.
 
  
 
~translated into English by Tomas Diaz. ~orders@tomasdiaz.com
 
~translated into English by Tomas Diaz. ~orders@tomasdiaz.com
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== See also ==
 
== See also ==
https://www.oma.org.ar/enunciados/ibe18.htm
 

Latest revision as of 04:38, 14 December 2023

Problem

The integers from 1 to 2002, both inclusive, are written on a blackboard in increasing order $1, 2, \cdots , 2001, 2002$. Then, those in first, fourth place, seventh place, etc. are erased, that is, those who occupy the places of the form $3k + 1$. In the new list the numbers that are in the places of the form $3k + 1$ are erased. This process is repeated until all numbers are deleted from the list. What was the last number that was erased?

~translated into English by Tomas Diaz. ~orders@tomasdiaz.com

Solution

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See also