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. Announce that you will lay three cards on the table, and deal them face up from the top of the deck. 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.
You'd add three cards from the other pile. This is where the math comes in. 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.