area_K = data.groupby('rooms').agg({'living_area' : 'mean', 'total_area' : 'mean'})

area_K['K'] = area_K['living_area']/area_K['total_area']



def fill_living_area(row):
    rooms = row['rooms']
    total_area = row['total_area']
    living_area = row['living_area']

    if rooms in area_K.index:
        return area_K.loc[rooms, 'K']*total_area
    return living_area

data.loc[data['living_area'].isnull(),'living_area'] = data.apply(fill_living_area, axis = 1)