The pet store has 6/2 = <<6/2=3>>3 cats for sale.
They have 6 x 2 = <<6*2=12>>12 birds for sale.
They have 6 x 3 = <<6*3=18>>18 birds for sale.
Therefore, there are a total of 6 + 3 + 12 + 18 = <<6+3+12+18=39>>39 pets for sale.
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