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How many 20-digit numbers are such that the sum of their digits is even?

 May 15, 2015

Best Answer 

 #1
avatar+111438 
+15

 

 

Half of them will have digit sums that are even.

 

And half of the number of 20 digit numbers =

  

(99,999,999,999,999,999,999 - 10,000,000,000,000,000,000 +1) / 2  =

 

45,000,000,000,000,000,000

 

 

 May 15, 2015
 #1
avatar+111438 
+15
Best Answer

 

 

Half of them will have digit sums that are even.

 

And half of the number of 20 digit numbers =

  

(99,999,999,999,999,999,999 - 10,000,000,000,000,000,000 +1) / 2  =

 

45,000,000,000,000,000,000

 

 

CPhill May 15, 2015
 #2
avatar+110206 
+5

Thanks Chris  :)

 

You know I can never remember this formula off the top of my head :(

 May 17, 2015

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