Greg and I were in a used bookstore last week and overheard the following conversation:
Customer: How much is this book?
Proprietor: All books are half priced.
Customer: So how much is this book?
Proprietor: All books are half of the original price.
Customer (looking at the book): The original price on this book is $9.99. How much would this book be?
Proprietor: Well, the price of the book would be half of $10.
Customer: So how much would that be?
Proprietor (more than a little curtly): You take $10 and divide by two, so $5.
I wonder how often the proprietor has that conversation. I do appreciate he tried to make it a teaching moment, with the whole "divide by two" instruction. That reminds me of a conversation a colleague had with a student this week, which went something like:
Student: How big is Mars?
Professor: It's about half the diameter of the Earth.
Student: What do you mean?
Professor: Mars is about half of the diameter of the Earth.
Student: I don't understand you.
Professor: You take the Earth's diameter and you multiply it by 0.5.
Student: Why didn't you just say so?