Johannes was able to sell $30 / $2 = <<30/2=15>>15 kilograms last Wednesday.
He was able to sell $24 / $2 = <<24/2=12>>12 kilograms last Friday.
And he was able to sell $42 / $2 = <<42/2=21>>21 kilograms today.
Therefore, Johannes was able to sell is 15 + 12 + 21 = <<48=48>>48 kilograms of cabbage.
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