Playing Bridge

While playing bridge , a hand of 13 random cards is dealt to you. What is the probability that you receive (strictly) more Kings than Queens?

Details and Assumptions


The answer is 0.341.

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1 solution

The probability A A of receiving more Kings than Queens is the same as the probability of receiving more Queens than Kings. The only other option is to have the same number of Kings and Queens; let this option occur with probability B B .

We then have that 2 A + B = 1 A = 1 B 2 2A + B = 1 \Longrightarrow A = \dfrac{1 - B}{2} .

So we now need to calculate B B . (The reason I'm taking this approach is that there are 10 10 cases to deal with to calculate A A and only 5 5 to deal with to calculate B B .)

Now we can have n n of both Kings and Queens in ( 4 n ) ( 4 n ) ( 44 13 2 n ) \dbinom{4}{n} \dbinom{4}{n} \dbinom{44}{13 - 2n} ways, where 0 n 4 0 \le n \le 4 . We thus have that

B = n = 0 4 ( 4 n ) 2 ( 44 13 2 n ) ( 52 13 ) = 0.316151778...... B = \dfrac{\displaystyle\sum_{n=0}^{4} \binom{4}{n}^{2} \binom{44}{13 - 2n}}{\dbinom{52}{13}} = 0.316151778...... ,

which in turn gives us A = 1 B 2 = 0.3419 A = \dfrac{1 - B}{2} = \boxed{0.3419} to 4 4 decimal places.

Yes that is the solution I intended to post.. This is the clever way and apparently the best too.

Pratik Shastri - 6 years, 7 months ago

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I'm glad we get three attempts on problems, since I only realized this approach on my third attempt. :)

Brian Charlesworth - 6 years, 7 months ago

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Did the same thing.

Hail calculators, though.

A Former Brilliant Member - 5 years, 7 months ago

I din't get that summation part. How to solve that to get the value of B as 0.316 So can anyone explain that part please.

Ishan Kothari - 6 years, 7 months ago

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Nothing special. Put the five terms along with the denominator into a calculator.

Pratik Shastri - 6 years, 7 months ago

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