I also uploaded the file for your question here. 

So read it carefully and if those explanation is still not enough, 

then i would like you to review some simple stoichiometry questions in my SAT chem lectures or in any other sources in google or youtube. 

So when you got an understanding you will see how those numbers beutifully work and not confusing at all. 😊

They are just mathematics in chemistry. 

I wish you good luck! 

See you

Kelda