It is written in Introduction section of book (xxvi):
If you can’t answer at least 80 percent of these questions correctly, go back and review the chapter, or at least those sections that seem to be giving you difficulty.
All of my chapter review questions results are less than 80 percent. Therefore I have decided to read all chapter again and note important parts of topics. So if I see any unclear point or typo (which I guess) I will share it.
I guess there is a typo on page 57, in the second sentence.
The result of x * y will then be automatically promoted to a double, so that it can be multiplied with z, resulting in a double value.