I got WA #4 with such code:

#include <cstdio>
#include <iostream>
#include <cmath>

using namespace std;

int main()
{
    int n;
    double a, sum = 0.0;

    scanf("%d", &n);
    for (int i = 0; i < n; i++)
    {
        scanf("%le", &a);
        sum += a;
    }

    printf("%.15e", sum / n);

    return 0;
}

WA #5 with such code:

#include <cstdio>
#include <iostream>
#include <cmath>

using namespace std;

int main()
{
    int n;
    double a, sum = 0.0;

    scanf("%d", &n);
    for (int i = 0; i < n; i++)
    {
        scanf("%le", &a);
        sum += a;
    }

    printf("%.7e", sum / n);

    return 0;
}

WA #6 with such code:

#include <cstdio>
#include <iostream>
#include <cmath>

using namespace std;

int main()
{
    int n;
    double a, sum = 0.0;

    scanf("%d", &n);
    for (int i = 0; i < n; i++)
    {
        scanf("%le", &a);
        sum += a;
    }

    printf("%.8e", sum / n);

    return 0;
}

AC with such code:

#include <cstdio>
#include <iostream>
#include <cmath>

using namespace std;

int main()
{
    int n;
    double a, sum = 0.0;

    scanf("%d", &n);
    for (int i = 0; i < n; i++)
    {
        scanf("%le", &a);
        sum += a;
    }

    printf("%.15e", sum / n);

    return 0;
}

But, connecting to the problem statements, "The offset should be printed with at least six digits after decimal point."