基數詞在用來表示準確數字的情況下不能加復數后綴"s"。以下基數詞可以加復數后綴"s"，因為它們表達的都是大概的數字。

1.thousands of / millions of / hundreds and hundreds of / hundreds of thousands of / thousands upon thousands of 成千上萬……

例：There are hundreds of thousands of people in the park. 公園里的人有千百個。

2. in twos and threes 三三兩兩

例：They came home from church by twos and threes. 他們三三兩兩地從教堂回來。

3. at sixes and sevens 亂七八糟

例：His wife was away and the house was at sixes and sevens. 他妻子不在，家里亂七八糟的。

4. in one’s fifties 在五十多歲時(fifties 前可插入 early, late, mid-, middle)

例：The boys are all in their teens. 那些男孩年紀都在十幾歲。

Jane is only in her early forties. 簡才剛剛四十出頭。

She was in her mid-twenties. 她有二十五、六歲。

He was then in his late seventies. 他當時已經快八十歲了。

5. in the 1960s (in the 1960’s) 二十世紀六十年代

例：He made acquaintance of her in the 1960s. 六十年代他認識了她