Mickey7Kid gave a reasonable answer without all the math bullshit
lugalWhy would you ask “How is this possible” when you expect the answer to be “it’s not”?
Because they spent an entire math class period earlier that week explaining to the students what "reasonableness" was going to mean on their next math test, and in the context of (I'm guessing 3rd or 4th grade) arithmetic the important thing they're trying to teach is that 5/6 is a larger fraction than 4/6. I agree that the question could be worded better (change the last two sentences to "Marty says he ate more pizza. Is this possible?") but I strongly suspect that the missing context from their class - or maybe even at the beginning of the test - explains enough to get the answer the teacher was looking for here.
Yes, one kid starting with a larger pizza changes the situation, but fundamentally that's an algebra question, not a "learning fractions" question.
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.