基数词在用来表示准确数字的情况下不能加复数后缀"s"。以下基数词可以加复数后缀"s"，因为它们表达的都是大概的数字。

1.thousands of / millions of / hundreds and hundreds of / hundreds of thousands of / thousands upon thousands of 成千上万……

例：There are hundreds of thousands of people in the park. 公园里的人有千百个。

2. in twos and threes 三三两两

例：They came home from church by twos and threes. 他们三三两两地从教堂回来。

3. at sixes and sevens 乱七八糟

例：His wife was away and the house was at sixes and sevens. 他妻子不在，家里乱七八糟的。

4. in one's fifties 在五十多岁时（fifties 前可插入 early, late, mid-, middle）

例：The boys are all in their teens. 那些男孩年纪都在十几岁。

Jane is only in her early forties. 简才刚刚四十出头。

She was in her mid-twenties. 她有二十五、六岁。

He was then in his late seventies. 他当时已经快八十岁了。

5. in the 1960s （in the 1960's） 二十世纪六十年代

例：He made acquaintance of her in the 1960s. 六十年代他认识了她。