You never set bool Even to false, anything other than 0 is true.

#include "stdafx.h"

bool Even( int );

int main()
{
if( Even(21) ) {
bool Even = true;
}

if (Even) {
std::cout << "Number is even!\n";
}
return 0;
}
bool Even( int nbr )
{
bool answer;
if ((nbr % 2) == 0)
answer = true;
else
answer = false;
return answer;
}