50 x 2/5 = <<50*2/5=20>>20 posters are small.
And 50/2 = <<50/2=25>>25 posters are medium.
So 20 + 25 = <<20+25=45>>45 posters are not large.
Therefore, 50 - 45 = <<50-45=5>>5 posters are large.
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