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A prime number less than 100 is selected randomly. What is the probability that its square has units digit 7?

 Sep 9, 2020
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These are the squares of ALL prime numbers below 100:

 

(4, 9, 25, 49, 121, 169, 289, 361, 529, 841, 961, 1369, 1681, 1849, 2209, 2809, 3481, 3721, 4489, 5041, 5329, 6241, 6889, 7921, 9409)

 

As you can see, there isn't a single one with its units digit = 7. Therefore, the probability is zero!.

 Sep 9, 2020

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