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a is chosen from the set {1,2,3,4,5}, and b is chosen from the set {5,6,7,8,9}. What is the probability that the product ab is a prime number?

 Feb 11, 2021
 #1
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a is chosen from the set {1,2,3,4,5}, and b is chosen from the set {5,6,7,8,9}. What is the probability that the product ab is a prime number?   

 

There can be 5 x 5 products.                         25 possible  

 

For the product to be a prime numer,

a must be 1 and b must be either 5 or 7.          2 possible  

 

Probability of the product being a prime is        2 / 25.  

 Feb 11, 2021
 #2
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Correcting my error in the above. 

I just realized, there's a duplicate. 

2x9 and 3x6 are the same product.  

 

So the probability is 2 / 24 which reduces to 1 /12

Guest Feb 11, 2021

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