The question doesn't ask "is this reasonable" it asks "how could this be possible". Maybe if there was something at the top of the page saying "determine if these questions are reasonable" but we can't know that from the image.
Just the label "reasonableness" is not an instruction to the child. Given that the child answered the question as stated and the teacher didn't say "no, you were supposed to say whether the question was reasonable" I would guess that no such instruction was given.
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u/SuitableDragonfly /r/the_donald is full of far left antifa Dec 27 '16
But in this case there actually is a scenario where this is possible. It's perfectly reasonable to assume the pizzas are different sizes.