@JKxZ In Magic: The Gathering, casting a Fireball with enough power to deal 20 damage to your opponent depends on various factors, such as the mana available and the cards in your hand or on the battlefield. To cast a Fireball, you need to have enough red mana sources to pay for its casting cost. If you have a sufficient amount of red mana available, you can cast Fireball for X damage, where X is the amount of mana you choose to spend on X during its casting. For example, if you have 10 red mana available, you can cast Fireball for X=10, dealing 10 damage to your opponent. If you want to deal 20 damage, you would need to have 20 red mana available. The speed at which you can accumulate mana depends on the cards in your deck and the strategies you employ. Some decks may be able to generate large amounts of mana quickly, while others may require more time to build up the necessary resources. It's important to note that Magic: The Gathering is a highly complex game with countless variables, so the specific answer to your question may vary based on the specific circumstances of your game.