Seeing three toilets that hadn't been flushed in one day was too much. Isn't it common sense to flush a toilet when you're finished?
この場合、次のような言い方ができますよ。
ーSeeing three toilets that hadn't been flushed in one day was too much. Isn't it common sense to flush a toilet when you're finished?
「1日に流されてないトイレを3個も見るのは多すぎる。終わったらトイレを流すのが常識じゃない?」
to flush a toilet「トイレを流す」
common sense「常識」
ご参考まで!