Cards Conditional

A deck of 10 cards numbered 1 through 10 is shuffled, and a card is drawn from the deck. Let A A be the event that an odd number is drawn and B B be the event that a prime number is drawn. What is P ( B A ) P(B\mid A) ?

3 4 \dfrac{3}{4} 2 5 \dfrac{2}{5} 1 2 \dfrac{1}{2} 3 5 \dfrac{3}{5}

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2 solutions

Andy Hayes
Jun 6, 2016

Relevant wiki: Conditional Probability

A = { 1 , 3 , 5 , 7 , 9 } A=\{1,3,5,7,9\}

B = { 2 , 3 , 5 , 7 } B=\{2,3,5,7\}

A B = { 3 , 5 , 7 } A\cap B=\{3,5,7\}

The sample space is uniform, so: P ( B A ) = A B A = 3 5 P(B\mid A)=\dfrac{|A\cap B|}{|A|}=\boxed{\dfrac{3}{5}}

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adolph damas - 3 years, 4 months ago

But 1 is not considered to be a prime number now!

Pacco Ortiz - 3 years ago

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1 is odd, not prime

Aisthu Lucky - 2 years, 7 months ago

yes, exactly. One (1) is not considered to be a prime! (But B is the subset for primes in this example) https://blogs.scientificamerican.com/roots-of-unity/why-isnt-1-a-prime-number/

Rch Pmp - 5 months, 4 weeks ago
Justin Franecki
May 2, 2018

Primes = { 1 ,2, 3 , 5 , 7 } = 5

Odds = { 1 , 3 , 5 , 7 ,9} = 5

Prime and Odd = { 1 , 3 , 5 , 7 } = 4

P r i m e s + O d d s P r i m e A n d O d d T o t a l N u m b e r O f C a r d s \frac{|Primes| + |Odds| - |Prime And Odd|}{|Total Number Of Cards|} = 5 + 5 4 10 \frac{5+5-4}{10} = 6 10 \frac{6}{10} = 3 5 \frac{3}{5}

Is 1 a prime number?

Pratik Jain - 2 years, 10 months ago

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Wiki says no, but 2 is a prime.

Микола Моргун - 2 years, 10 months ago

I guess correct numerator is |PrimeAndOdd|.

Микола Моргун - 2 years, 10 months ago

1 is not a prime number - hence this solution is wrong ( https://uk.answers.yahoo.com/question/index?qid=20090904184713AA9yBs2&guccounter=1 )

Spar Ky - 2 years, 8 months ago

What you're representing is not AND, that's exclusive OR = XOR A+B-(A&B).

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