The mana value of a double-faced card in your graveyard is always the mana value of the front face.

X is 0 when determining the mana value of a card in your graveyard.

The mana value of a split card in the graveyard is the total mana value of both halves of that card. It does not have two mana values.

The mana value of a land card is 0.

“Five or more mana values among cards in your graveyard” means there are at least five different mana values among those cards.