You're assuming those 40 hours are used all productively. I don't think it's wildly off to assume that 8 hours of the 40 hour week are basically wasted because

You're assuming those 40 hours are used all productively. I don't think it's wildly off to assume that 8 hours of the 40 hour week are basically wasted because people like playing around once their concentration is gone. Plus on friday, people tend to be a lot less productive.