
Here's how it worked:
The companionship would trade off doors. Each door would be affixed a point value based on the final number in the address. The value for apartment 32 would be 2, the value for 123 Main Street would be 3. But, if someone answers the door, the value doubles. Going door to door, the missionaries add the values of their doors with the objective being to get as close to 21 without going over. At any point a missionary can elect to hold which then transfers the door in play and any subsequent door to his companion.
If, at any time, someone behind a door invited us in, the missionary's score immediately jumped to 21 and he won that game.
Inevitable, the game ended with my companion or myself secretly hoping that no one would be home at a door we were tracting, which is why tracting 21 (not to mention tracting bingo) isn't taught in the MTC.
(Read another strip at deseretnews.com.)
*Outside the Church, I suppose this would be known as "tracting blackjack".