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Try to create a subset from {1,2,3...,2019} so that in the subset every two numbers' sum is a multiple of 20. What is the maximum size of this subset?

 

-----------Thanks! laugh

 

Ummm.... accidentally pressed off-topic... oops!

 Mar 25, 2020
edited by madyl  Mar 25, 2020
edited by madyl  Mar 25, 2020
 #1
avatar+111330 
+2

Note  that  

 

{ 1, 2019 , 2, 2018, 3, 2017.....}   will create summed pairs  = 2020  =   101 (20)

 

The only number  that will not  have a "partner"  is  1010

 

So....the  max size of this  subset =  2018

 

cool cool cool

 Mar 26, 2020
edited by CPhill  Mar 26, 2020

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