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!
Ummm.... accidentally pressed off-topic... oops!
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
+288 
