Rule 708.4b

The mana cost of a split card is the combined mana costs of its two halves. A split card’s colors and converted mana cost are determined from its combined mana cost. Example: Assault/Battery’s mana cost is {3}{R}{G}. It’s a red and green card with a converted mana cost of 5. If you cast Assault, the resulting spell is a red spell with a converted mana cost of 1.

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