Monday, April 29, 2019

Solitaire: Chance for Starting Duplicates

In Klondike Solitaire, what's the chance for starting with two face-up cards of the same rank & color (what we might call "shadows", cards which don't help us very much)?

There are 7 face-up cards in starting Solitaire, so we consider the ways to choose 7 cards randomly without any duplicates. Every time we select such a card, the number available for the next draw (avoiding "shadows") goes down by 2, while the cards in the total deck goes down by 1. Therefore the chance for no-shadows is:
$${52 \over 52} \times {50 \over 51} \times {48 \over 50} \times {46 \over 49} \times {44 \over 48} \times {42 \over 47} \times {40 \over 46} \approx 0.63$$
This in turn means that the complement event, i.e., getting any shadows at all (at least two cards with duplicate rank & color),has a probability of:
$$1 - 0.63 = 0.37 = 37 \%$$
That is: About one chance in three.

More about Solitaire. 

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