twenty one还是twenty-one 数字中什么时候加-
问题描述：
twenty one还是twenty-one 数字中什么时候加-
答
做名词时,都不加-.比如,he is twenty one.他21岁.
做形容词,做定语,放在宾语后边,不加-.比如,Now,we study lesson twenty one.
做形容词,做定语,放在宾语前边,加-.比如.a twenty-one year old girl.