How do you think estimation is used in determining sale prices?

Marla bought a book for $12.95, a binder for $3.49, and a backpack for $44.99. the sales tax rate is 6%. find the amount of tax and the total she paid for these items

Ammonia molecules have three hydrogen atoms and one nitrogen atom.how many of each atom arein five molecules of ammonia

Ineed to know the answer to this algebra question.