Look at a card like {entreat the Angels}. It has 3 white and xx in its mana cost. What does xx mean?
Pay any amount of mana, that's even. Then, it's divided by 2, since Both X's need to be the same.
That's what I thought, but I couldn't be sure. Thanks!
Basically whenever you pay X, it means you choose a number and pay X mana right? Like for {Black Sun's Zenith}, you would choose 3 as X, so pay 3 mana for the X in the mana cost.
It's the same way for XX spells. You still only choose a number once. For instance, to cast {Bonfire of the Damned} or {Entreat the Angles}, it costs XX. So you choose any number, and substitute it for X wherever it appears. So let's put this into practice.
It's late game, and you have been holding an {Entreat the Angels} since the darn thing was in your opening hand. Your opponent is at 12 life, and could topdeck a burn spell to kill you any time now! You decide it's time for those angels to be entreated. He is at 12, so 3 angels ought to do the trick!
The mana cost is WWWXX. 3 is what you chose, so the mana cost is now WWW33. Not 33, but two seperate 3s. You could also picture it as this: WW3W3. So it is clear now that the total mana cost will be 9.