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fifteen; 15
「十五」 has another reading with different meanings.
クラスの生徒は全部で十五人いる。
There are fifteen students in the class in total.
十五分ほど待てば次のバスが来る。
If you wait about fifteen minutes, the next bus will come.
十五歳の時に初めて海外に行った。
I went abroad for the first time when I was fifteen.