Hi Su1206,Great question! Let's work through the probability of Mardea winning (scoring
3 or more points).
First, let's figure out the deck composition from the diagram. There are
10 cards total:
3 'draw again' cards,
4 cards worth
2 points,
2 cards worth
3 points, and
1 card worth
4 points.
Path 1: She draws a scoring card first (probability
7/10). She wins if she scores
3 or
4 points:
7/10 × (
2/7 +
1/7) =
7/10 ×
3/7 =
3/10.
Path 2: She draws 'draw again' first (probability
3/10). That card is removed, leaving
9 cards:
2 draw-again,
4 two-pointers,
2 three-pointers,
1 four-pointer. From here she can score
3+ directly with probability
3/9, or draw again (
2/9) and face a similar situation with
8 cards.
You could chase each sub-path, but here's the elegant shortcut:
Key Insight: The 'draw again' cards simply delay Mardea until she eventually draws a scoring card. They get removed one by one without affecting the scoring cards at all. Since the deck always contains exactly
4 two-point cards and
3 winning cards (
2 three-point +
1 four-point), the ratio of winning cards to total scoring cards is always
3/7.
Note that no matter how many 'draw again' cards she cycles through, she is guaranteed to eventually draw a scoring card, and the
probability that scoring card is worth
3+ points is always
3 out of
7.
So the probability that Mardea wins = 3/7, which is approximately 42.9%.You can verify: P(win) =
7/10 ×
3/7 +
3/10 × (
7/9 ×
3/7 +
2/9 × (
7/8 ×
3/7 +
1/8 ×
3/7)) =
3/10 +
3/10 ×
3/7 + ... =
3/7. Every path simplifies to
3/7.
Answer: 3, 10