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May 26, 2025
May 25, 2025
May 24, 2025
 #2
avatar+15 
+1

We will calculate each of the probabilities step by step using combinations and the structure of a standard deck of 52 playing cards.

Total possible 5-card hands:

The total number of ways to choose 5 cards out of 52 is:

(525)=2, ⁣598, ⁣960\binom{52}{5} = 2,\!598,\!960(552​)=2,598,960

(a) Probability of a Straight

A straight is 5 cards in sequence (e.g. A-2-3-4-5, 2-3-4-5-6, ..., 10-J-Q-K-A), not all the same suit (if they are, it's a straight flush). We ignore suit and order.

Step 1: Count possible ranks for a straight

There are 10 possible sequences of 5 consecutive ranks (starting with A-2-3-4-5 up to 10-J-Q-K-A).

Step 2: For each sequence, count suit combinations

Each card in the 5-card sequence can be of any of the 4 suits.

So number of ways to assign suits to the 5 cards:

45=10244^5 = 102445=1024

But we exclude straight flushes, where all 5 cards are of the same suit:
There are 4 straight flushes for each of the 10 sequences, so:

10×4=40 straight flushes10 \times 4 = 40 \text{ straight flushes}10×4=40 straight flushes

So total number of non-straight-flush straights:

10×(45−4)=10×(1024−4)=10×1020=10, ⁣20010 \times (4^5 - 4) = 10 \times (1024 - 4) = 10 \times 1020 = 10,\!20010×(45−4)=10×(1024−4)=10×1020=10,200

Final Probability:

\frac{10,\!200}{2,\!598,\!960} \approx 0.0039 \quad \text{(or about 0.39%)}

(b) Probability of a Flush

A flush is 5 cards of the same suit, not in sequence.

Step 1: Choose a suit

There are 4 suits.

Step 2: Choose any 5 cards from the 13 cards in that suit:

(135)=1287\binom{13}{5} = 1287(513​)=1287

So total flushes (including straight flushes):

4×1287=51484 \times 1287 = 51484×1287=5148

Now subtract the 40 straight flushes (from part a):

5148−40=51085148 - 40 = 51085148−40=5108

Final Probability:

\frac{5108}{2,\!598,\!960} \approx 0.001965 \quad \text{(or about 0.197%)}

(c) Probability of a Full House

A full house is 3 cards of one rank and 2 cards of another rank.

Step 1: Choose rank for the triplet

There are 13 ranks, choose 1:

(131)=13\binom{13}{1} = 13(113​)=13

Choose 3 cards of that rank from 4 suits:

(43)=4\binom{4}{3} = 4(34​)=4

Step 2: Choose rank for the pair (different from triplet)

There are 12 remaining ranks:

(121)=12\binom{12}{1} = 12(112​)=12

Choose 2 cards of that rank:

(42)=6\binom{4}{2} = 6(24​)=6

Total number of full houses:

13×4×12×6=374413 \times 4 \times 12 \times 6 = 374413×4×12×6=3744

Final Probability:

\frac{3744}{2,\!598,\!960} \approx 0.001440 \quad \text{(or about 0.144%)}

Final Answers:

(a) ~0.0039 (0.39%)
(b) ~0.001965 (0.197%)
(c) ~0.001440 (0.144%)

May 24, 2025
May 23, 2025
 #1
avatar+114 
0

Let S={0,1,…,9}. We are looking for subsets of S that have at least two elements, and the sum of the two largest elements in the subset is 4.

Let the subset be A⊆S. Let the two largest elements in A be a and b, with a>b. We require a+b=4.

Let's list the possible pairs (a,b) such that a>b and a+b=4:

If a=4, then b=0. The pair is (4,0).

In this case, 4 and 0 must be in the subset.

All other elements in the subset must be less than b=0. However, there are no elements in S less than 0.

So, the only subset for this case is {4,0}. This subset has two elements, and 4+0=4. So, this is a valid subset. (1 subset)

If a=3, then b=1. The pair is (3,1).

In this case, 3 and 1 must be in the subset.

All other elements in the subset must be less than b=1. The only element in S less than 1 is 0.

So, for this pair, the subset must contain {3,1}. We can optionally include 0.

The possible subsets are:

{3,1}

{3,1,0}

Both of these subsets have at least two elements, and the two largest elements are 3 and 1, with a sum of 4. (2 subsets)

If a=2.5, not possible as elements are integers.

Notice that a must be at least 2. The largest possible value for a is 4 (since a+b=4 and b must be at least 0).

Also, b must be at least 0.

And a>b.

Let's re-examine systematically:

The two largest elements, a and b (a>b), must sum to 4.

Possible pairs (a,b) from S satisfying a+b=4 and a>b:

Case 1: a=4,b=0

The subset must contain {4,0}.

All other elements in the subset must be smaller than b=0. There are no such elements in S.

So, the only subset is {4,0}. This subset has two elements, and the sum of its two largest elements (4 and 0) is 4.

Number of subsets for this case: 1.

Case 2: a=3,b=1

The subset must contain {3,1}.

All other elements in the subset must be smaller than b=1. The only element in S smaller than 1 is 0.

So, the subset must contain {3,1}, and we can either include or exclude 0.

The possible subsets are {3,1} and {3,1,0}.

Both of these subsets have at least two elements, and the sum of their two largest elements (3 and 1) is 4.

Number of subsets for this case: 2.

We cannot have a=2 because if a=2, then b=2, but we require a>b.

Therefore, the total number of subsets satisfying the given properties is the sum of the subsets from each case:

Total subsets = (Subsets from Case 1) + (Subsets from Case 2)

Total subsets = 1 + 2 = 3.

The subsets are:

{4,0}

{3,1}

{3,1,0}

The final answer is 3​.

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