Annie starts with 21 + 36 = <<21+36=57>>57 crayons.
Bobby ends up giving her 36 / 2 = <<36/2=18>>18 crayons.
Annie now has a total of 57 + 18 = <<57+18=75>>75 crayons.
She ends up giving Mary a total of 75 * (1 / 3) = <<75*(1/3)=25>>25 crayons.
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