The correct answers are: Helmer knows that there is a piece of mail from Krogstad in the box. Nora looks at her watch and tells herself she has 31 hours to live. Helmer thinks that Nora is nervous and dismisses her pleas about the mail.
Indeed, the text is both very explicit and implicit. Helmer’s character explicitly declares that he thinks a letter from Krogstad is inside his locked mailbox. Nora is very explicit about having only 31 hours to live since Helmer will discover the latter and she will take her own life due to that.
The implicit part is that Helmer, who is a very condescending chauvinist, does not take her nervousness seriously and treats her as a child that is going through one of her phases. He will read the letter whenever he wants and the only way Nora is able to keep him from doing that is to keep him busy with her until the deadline arrives.
