C#

public ArrayList FindPrimes(int maxNumber) 
{
	ArrayList result = new ArrayList();
	for (int i = 3; i <= maxNumber; i += 2)
		if (IsPrime(i))
			result.Add(i);
	return result;
}

public double GetPrimesAverage(int maxNumber)
{
	return CalcAverage(FindPrimes(maxNumber));
}

public bool IsPrime(int number) 
{
	double max = Math.Sqrt(number);
	for (int i = 3; i <= max; i += 2)
		if (number % i == 0)
			return false;
	return true;
}

public double CalcAverage(ArrayList list) 
{
	double sum = 0;
	foreach (int number in list)
		sum += number;
	return sum / list.Count;
}

private void listBox1_MouseUp(object sender, System.Windows.Forms.MouseEventArgs e)
{
	if (e.Button == MouseButtons.Right) 
	{
		int index = listBox1.IndexFromPoint(e.X, e.Y);
		if (listBox1.SelectedIndices.Contains(index))
			contextMenu1.Show(listBox1, new Point(e.X, e.Y));
	}
}