YOU SAID:
Their were fifteen cats at the party.
INTO JAPANESE
彼らはパーティーで15匹の猫でした。
BACK INTO ENGLISH
They were 15 cats at the party.
INTO JAPANESE
彼らはパーティーで15匹の猫でした。
BACK INTO ENGLISH
They were 15 cats at the party.
That didn't even make that much sense in English.