A woman is having some trouble(患病) with her eyes, so she goes to see the doctor. He is a new doctor and doesn’t know her, so he asks some questions and one of them is, “How old are you?”

     The woman says, “I don’t know, doctor, but let me think about it.” She thinks for a few minutes and then says, “Yes, I know now, doctor! When I marry(结婚), I am eighteen years old, and my husband is thirty. Now my husband is sixty. And that is twice(两倍) thirty. So I am twice eighteen. That’s thirty-six, isn’t it?”

10张面值5角可以换____张1块纸币。