Rule 702.1b

An effect that grants an object a keyword ability may define a variable in that ability based on characteristics of that object or other information about the game state. For these abilities, the value of that variable is constantly reevaluated. Example: Volcano Hellion has the ability “Volcano Hellion has echo {X}, where X is your life total.” If your life total is 10 when Volcano Hellion’s echo ability triggers but 5 when it resolves, the echo cost to pay is {5}. Example: Fire//Ice is a split card whose halves have the mana costs {1}{R} and {1}{U}. Past in Flames reads “Each instant and sorcery card in your graveyard gains flashback until end of turn. The flashback cost is equal to its mana cost.” Fire//Ice has “Flashback {2}{U}{R}” while it is in your graveyard, but if you choose to cast Fire, the resulting spell has “Flashback {1}{R}.”

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