回答

(1) This skirt is 10% less expensive/cheaper than usual. I’ll buy it.
※比較級 less expensive/cheaper はどちらでもOK than usual いつもより
(2) The world's population today is about four times that of 100 years ago.
※that(代名詞) = the world' population(世界の人口)
(3) Oil has played one of the most important roles in human lives.
※play a role 役割を果たす
(4) Dr. Tanaka discovered that the ruins were far older than he had expected.
※far+比較級… はるかに…
(5) Due to the widespread use of email, people write fewer letters than they used to.
※due to … …のために<理由・原因> widespread use 広範な使用 used to ~ 昔よく~したものである<過去の習慣>

参考にしてください。

taka

UMIさん、(2) The world's population today is about four times as large as that of 100 years ago.が正解ですので、すみませんが、訂正をお願いします。

げすとさん、ご指摘ありがとうございます。

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