If there are four girls and eight boys in your class, you may say there are two times as many boys than girls in my class. 如果你班上有四个女生和八个男生，你可能会说我班上的男生是女生的两倍。

　　This expression uses the ratio. 这个表达用到了比率。

　　A ratio does not tell you the number of all the students, which is the sum of the two groups. 比率并没有告诉你所有学生的人数，即两组的人数总和。

　　However, it explains the relationship between the number of boys and the number of girls. 但是，它解释了男生数量和女生数量之间的关系。

　　When we express the ratio, we use the colon mark. 当我们表示比率时，我们用冒号。

　　For example, the ratio of girls to boys in the example above is 1:2. 例如，在上面的例子中，女生与男生的比例是1：2。

　　It means there are two boys for every one girl. 这代表着两个男生对应一个女生。

　　A ratio is very important to understand the characteristics of a large group. 比率对于理解一个大群体的特征非常重要。