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How many two-digit numbers have non-zero digits whose sum is odd? For example,54and38.

 Jan 31, 2021
 #1
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Two cases.....first digit  odd, second even

We have  5 choices for the  first  digit=  1,3,5,7,9

And we  have  4  choices for  the  second   2,4,6,8

Total  =  5 * 4   = 20

 

Second case......first digit  even,  second  odd

4 choices  for the  first digit  =2,4,6,8

5 choices for the  second  digit  = 1,3,5,7,9

So, again   ...20

 

 

So

 

20  +  20    =   40

 

 

cool cool cool

 Jan 31, 2021

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