First find the number of green marbles: 20 yellow marbles / 2 = <<20/2=10>>10 green marbles
Then find how many marbles are either red or blue: 60 marbles - 20 yellow marbles - 10 green marbles = <<60-20-10=30>>30 marbles
Divide this number in half to find how many marbles are blue: 30 marbles / 2 = <<30/2=15>>15 blue marbles
Finally, divide the number of blue marbles by the total number of marbles and multiply by 100 to find the percentage chance of getting a blue marble: 15 marbles / 60 marbles * 100 = <<15/60*100=25>>25%
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