Historical Rules · 2 February 2024
Rule 202.3f
Official source text from the 2 February 2024 Rules release
When calculating the mana value of an object with a hybrid mana symbol in its mana cost, use the largest component of each hybrid symbol. Example: The mana value of a card with mana cost {1}{W/U}{W/U} is 3. Example: The mana value of a card with mana cost {2/B}{2/B}{2/B} is 6.