SOLUTION: coin collector had 6 coins for sale one he price @15,16,18,19,20,31 he sold all but one of the coins to 2 people one payinng 2xs as much as the other one which coin did he ke

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I just tried every single number that could be the one left out
First I let = the amount one person paid and =
amount other person paid. , so the sum of
what they bought must be divisible by
The only way I could arrive at this is if the coin costing 20
is left out. Then , so if



1 person bought 33 worth of coins
The other bought 66 worth

and

So, he kept the coin costing 20