Card Probabilities
1. **Problem statement:** We deal 3 cards from a standard 52-card deck. Find the probabilities of:
(a) 2 of a kind (exactly two cards of the same rank),
(b) 3 of a kind (all three cards of the same rank),
(c) 3 cards of the same suit (a flush),
(d) 3 cards in consecutive order (a straight).
2. **Total number of 3-card hands:**
$$\text{Total} = \binom{52}{3} = \frac{52 \times 51 \times 50}{3 \times 2 \times 1} = 22100$$
3. **(a) Probability of exactly 2 of a kind:**
- Choose the rank for the pair: 13 choices.
- Choose 2 suits out of 4 for the pair: $\binom{4}{2} = 6$ ways.
- Choose the third card rank different from the pair rank: 12 choices.
- Choose the suit of the third card: 4 choices.
Number of favorable hands:
$$13 \times 6 \times 12 \times 4 = 3744$$
Probability:
$$P(2\text{-of-a-kind}) = \frac{3744}{22100} \approx 0.1694$$
4. **(b) Probability of 3 of a kind:**
- Choose the rank: 13 choices.
- Choose all 3 suits out of 4 for that rank: $\binom{4}{3} = 4$ ways.
Number of favorable hands:
$$13 \times 4 = 52$$
Probability:
$$P(3\text{-of-a-kind}) = \frac{52}{22100} \approx 0.00235$$
5. **(c) Probability of 3 cards of the same suit (flush):**
- Choose the suit: 4 choices.
- Choose any 3 cards from the 13 cards of that suit: $\binom{13}{3} = 286$ ways.
Number of favorable hands:
$$4 \times 286 = 1144$$
Probability:
$$P(\text{flush}) = \frac{1144}{22100} \approx 0.0518$$
6. **(d) Probability of 3 cards in consecutive order (straight):**
- Number of possible 3-card straights:
- Possible starting ranks: from Ace-2-3 up to Queen-King-Ace, counting Ace as low or high, total 12 sequences.
- Each rank in the straight has 4 suits, so total ways per sequence:
$$4^3 = 64$$
Number of favorable hands:
$$12 \times 64 = 768$$
Probability:
$$P(\text{straight}) = \frac{768}{22100} \approx 0.03475$$
**Final answers:**
- (a) $\approx 0.1694$
- (b) $\approx 0.00235$
- (c) $\approx 0.0518$
- (d) $\approx 0.03475$