Half of the marbles are white, so there are 50 / 2 = <<50/2=25>>25 white marbles.
Green marbles are only 50% as many as yellow marbles, so there are 50/100 * 12 = <<50/100*12=6>>6 green marbles.
So currently we know that we have 25 + 12 + 6 = <<25+12+6=43>>43 marbles.
The rest are red, so there are 50 - 43 = <<50-43=7>>7 red marbles.
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