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How many two-digit numbers have non-zero digits whose sum is prime?

 Aug 1, 2021
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How many two-digit numbers have non-zero digits whose sum is prime?     

 

I don't know a way to do this except by brute force. 

What "brute force" means, in mathematics, is that you just try everything. 

 

Like so: 

 

12, 14, 16 

21, 23, 25, 29 

32, 34, 38 

41, 43, 47, 49 

52, 56, 58 

61, 65, 67 

74, 76 

83, 85, 89 

92, 94, 98 

 

I stacked them like that to see if I could detect a pattern.  I couldn't. 

I made this list quickly in my head so I might've missed one/some. 

If I didn't overlook one/some, the total is 28.  Check my work.    

.

 Aug 1, 2021

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