def calculate_stats(numbers):
    mean = sum(numbers) / len(numbers)
    sorted_numbers = sorted(numbers)
    n = len(sorted_numbers)
    median = (sorted_numbers[n//2] + sorted_numbers[(n-1)//2]) / 2
    frequency = {}
    for num in numbers:
        frequency[num] = frequency.get(num, 0) + 1
    mode = max(frequency, key=lambda k: frequency[k])
    return mean, median, mode

# Example usage
data = [1, 2, 3, 4, 5, 5]
print(calculate_stats(data))  # Output: (3.5, 3.5, 5)