prog13_2.py

import csv

with open("population.csv", encoding="utf-8", newline="") as file_obj:
    reader_obj = csv.reader(file_obj)
    next(reader_obj)  # ヘッダ行を捨てる
    print("国 　平均（百万人）")
    for row in reader_obj:
        country = row[0]
        sum_ = 0.0
        for pop in row[-3:]:
            sum_ += float(pop)
        average = sum_ / 3
        print(f"{country}　{average:.1f}")

# プログラム13-2（国別平均人口）