9 and 10 year olds lack the understanding or ability adjust for differently sized wholes.

Ok, I misspoke when I said it’s not testing for reasonableness. It’s what I get for commenting right after waking up. The reasonableness it’s checking for is can the students understand what makes the situation true. That the wholes are different sized. It’s not a poorly worded problem, the teacher just doesn’t know what they are doing.

Source: again, I teach this scenario every year and the students figure it out every year.