question about combinatorical

Posted by davit-datuashvili on Stack Overflow See other posts from Stack Overflow or by davit-datuashvili
Published on 2010-06-13T07:32:13Z Indexed on 2010/06/13 7:42 UTC
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here is task

How many ways are there to choose from the set {1, 2, . . . , 100} three distinct
numbers so that their sum is even?

first of all sum of three numbers is even if only if

1.all number is even
2.two of them is odd and one is even

i know that

(n)   =  n!/(k!*(n-k)!   
(k)   

and can anybody help me to solve this problem

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