Half and Half
You have to be quick at addition and subtraction and a good memorizer to pull this off smoothly. First, let someone shuffle the deck to let them know you haven't set it in any way. Ask someone how many cards are in a deck without jokers. If they don't know that it's 52, tell them so. After this, ask someone what half of that is. If they don't know it's 26, this will stupefy them. Tell them you will count down 26 cards, and do so, face up, memorizing the fourth card. (This is important.)
Put the remaining cards aside and turn the face-up cards and hold them face-down. Announce that you will lay three cards on the table, and deal them face up from the top of the deck. Explain that each of those three cards has to be made equal to, or greater than, ten (Jack, Queen, King, and Ace are greater than ten) and that you will put cards on them to make them so. Nonchalantly put down the cards you're holding and pick up the other 26-card pile. Deal face down onto each face-up card the number of cards needed. (Say one of the cards is the seven of clubs. You'd add three cards from the other pile. If it's a ten, Jack, Queen, King, or Ace, you'd add nothing to it.)
This is where the math comes in. Place the cards you're holding on top of the other pile (from which you dealt the three cards), and total up how many cards you placed on the three face-up cards. Subtract that number from 26 and tell a participant to count down the number you get. It doesn't matter whether the cards are dealt face up or down. When the person is done, flip over the top card of the counted down pile if it isn't face up, then flip over the last card placed on the three face up cards, if there were cards placed on them. Pretend to be thinking for a second, then announce that the next card the person turns over is whatever you memorized as the fourth card when originally counting down.
If you're confident enough, you can let a participant count down the 26 cards, or look away when counting.