You could argue that it’s reasonable to assume that all pizzas are the same size but there are many pizza places that offer different sizes. You could as well argue that this is an attempt to make the kids think outside the box and come up with this explanation. How big a fraction is depends on how much the whole is is a good message you can’t learn too early. Understanding statistics is in large parts this. Many people will throw around percentages of pooling questions without ever questioning the pool of people asked.