faster implementation of sum ( for Codility test )

Here is my solution and I scored 100%

 public static int solution(int[] A)
    {
        double sum = A.Sum(d => (double)d);
        double leftSum=0;
        for (int i = 0; i < A.Length; i++){
            if (leftSum == (sum-leftSum-A[i])) {
                return i;
            }
            else {
                leftSum = leftSum + A[i];
            }
        }
        return -1;
    }

I don't think your problem is with the function that's summing the array, it's probably that you're summing the array WAY to frequently. If you simply sum the WHOLE array once, and then step through the array until you find the first equilibrium point you should decrease the execution time sufficiently.

int equi ( int[] A ) {
    int equi = -1;

    long lower = 0;
    long upper = 0;
    foreach (int i in A)
        upper += i;

    for (int i = 0; i < A.Length; i++)
    {
        upper -= A[i];
        if (upper == lower)
        {
            equi = i;
            break;
        }
        else
            lower += A[i];
    }

    return equi;
}