Any Way You Count 'Em
Shuffle the cards well. Holding them face down, turn over the top card and place it face up on the table. Think of it as a stack. Starting with its face value, deal face up on top of it as many more cards as needed to reach 10. For instance if it's a 3, deal seven cards on top of it; if it's a 5, deal five cards. Face cards count as 10, so no more cards are needed. An ace counts as 1 and needs nine more cards.
Continue making stacks as above, keeping them separate, until the deck
s exhausted. If there are not enough cards to complete a final stack, keep that incomplete stack in your hand. Now choose at random any three stacks that contain at least four cards each, and turn these stacks face down.
Gather all the remaining cards in any order and add them to the cards (if any) in your hand. Pick any two of the three face-down stacks on the table, and turn up the top card on each of those two piles. Add their values together. Discard that many cards from those in your hand, and then discard 19 additional cards.
Count the remaining cards in your hand. Now turn up the top card of the third stack. Its value will equal the number of cards in your hand.