Claire has 400 - 120 = <<400-120=280>>280 roses in her garden.
Thus, she has 280 - 80 = <<280-80=200>>200 red roses.
One-half of the red roses is equal to 1/2 x 200 = <<1/2*200=100>>100.
Therefore, Claire will earn $0.75 x 100 = $75 if she can sell 1/2 of the total number of red roses.
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