这个whom在这里是什么使用方法

Look at China,for example; they have 1.4 billion people, half a billion of whom have recentlyentered the middle class

Look at China, forexample; they have 1.4 billion people, halfa billion of whom have recently entered the middle class.

黑体部分是定语从句，1.4 billion people 是先行词。

1. 先行词是人，可以用 “数字 + whom” 引出定语从句，说明这类人中有多少......。

2. 先行词是物，可以用 “数字 + which”引出定语从句，说明这类物中有多少......。

She has threedaughters, one of whom is a doctor.

I have severalbooks, one of which is about cloning.