If Selma has fifty marbles, then Merill and Elliot together have 50-5 = <<50-5=45>>45 marbles.
We'll assume the number of marbles that Elliot has is x.
If Merill has twice as many marbles as Elliot, then the total number of marbles that Merill and Elliot have together is 2x+x = 45 marbles.
This translates to 3x= 45
The number of marbles that Elliot has is x = 45/3
This means Elliot has x = 15 marbles.
Since Merill has twice as many marbles as Elliot, Merill has 2*15 = <<2*15=30>>30 marbles.
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