Does "for every 5 cards in both players' trash" mean for every 5 cards in either mine or my opponent's trash?
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Official Ruling
No, it means every 5 cards in yours and your opponent's trashes combined. For example, if you have 12 cards in your trash and your opponent has 8 cards in theirs, that makes a total of 20 cards, so the play cost is reduced by 8.