def calc_centroid(bolts):
    """Calculate the centroid of the bolt group.
    
    Args:
        bolts (data struct): list of the bolt data structure

    Returns:
        x_centroid (float): x-coordinate of bolt group centroid
        y_centroid (float): y-coordinate of bolt group centroid

        This coordinate pair is returned as a tuple, as follows:
        (x_centroid, y_centroid)
    """
    sum_x = 0.0
    sum_y = 0.0
    num_bolts = len(bolts)
    for bolt in bolts:
        x = bolt[1][0]
        y = bolt[1][1]
        sum_x = sum_x + x
        sum_y = sum_y + y

    x_centroid = sum_x/num_bolts
    y_centroid = sum_y/num_bolts

    return x_centroid, y_centroid