Master of Predicaments

If the card is a split card, its mana value is determined by the combined mana cost of its two halves. If the player guesses wrong and you cast a split card this way, you may cast either half (or both halves if the card has fuse), no matter what its mana value is.

If you don't cast the card, either because the player guessed right or because you choose not to cast it, you don't have to reveal it. Your opponent won't know the reason you didn't cast the card.

You cast the card in your hand as part of the resolution of the triggered ability. Timing restrictions based on the card's type (such as creature or sorcery) are ignored. Other restrictions, such as "Cast [this card] only during combat," are not.