在10个球中有6个红球和4个白球（各不相同），不放回地依次摸出2个球，在第一次摸出红球的条件下，第2次也摸到红球的概率为（  ）

根据短文填表。

    Mrs. Wilson is not tall with curly hair. She comes from Australia. She is our math teacher. But I don't like maths. It's too difficult and boring. Mrs. Wilson is 25 years old. She is very shy and kind of serious. She never plays games with us. She often sees the films on weekends. And she enjoy reading very much. She has lots of books in her bedroom.

西北内陆的种植业分布零散的主要原因是（　　）

①在不等式的两边都加上（或减去）同一个整式，不等号的方向不变；

②在不等式的两边都乘同一个数（或整式），乘的数（或整式）为正时不等号的方向不变，乘的数（或整式）为负时不等式的方向改变．

请解决以下两个问题：

六年级学生人数是五年级学生人数的 $\frac{6}{7}$ ，六年级有学生420人，五年级有学生____人？