Math

 

Alex chooses a number at random from the set {1, 2, 3, 4, 5}.  Winnie also chooses a number at random from the same set.  (They can choose the same number.)  What is the probability that the product of their numbers is at least 4?     

 

The following are the 25 pairs that could occur.     

 

1 1    1 2    1 3    1 4    1 5     

2 1    2 2    2 3    2 4    2 5     

3 1    3 2    3 3    3 4    3 5     

4 1    4 2    4 3    4 4    4 5     

5 1    5 2    5 3    5 4    5 5     

 

The highlighted pairs are the 20 that multiply to at least four.     

The probability of choosing a highlighted pair is 20 / 25 which is 80%.     

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