First of all, X refers to the number of girls in 1 class.

4/5 means that the number of boys is 80% of the number of girls, and 80%=4/5.

And the title also says that the number of boys is 80% of the number of girls, so ×4/5.

But because it is, when a boy speaks on the stage, the number of boys in the audience is 80% of the number of girls, provided that when a boy speaks on the stage.

So add 1,

Because x×4/5+ 1 find 1 number of boys in the class;

And-1 in (x- 1) is because when a girl gives a speech on stage.

7/8 is because the number of boys in the audience is 8/7 of the number of girls.

Detailed enough, brother, please adopt, Baidu knows it is willing to answer for you! I wish you progress in your study!