Comment Results of extensive research in the UK into this: (Score 0) 1268
Ok, so this is not statistically meaningful, but from a sample size of two students (well, my kids) here:
The elder got the question in two seconds and filled the parenthesis with a 7 within 5. No help.
The younger couldn't "get" the question, even with some prompting, and filled the parenthesis with 11.
They are eight and six years' old; neither are especially mathematically talented, but put in more than average effort according to their teachers on that subject.
Both go to the local (non-private average-sized) school.
I'd think (i.e., place a small bet on it) around half the 7-to-9 year olds at that school would be able to "do" this problem correctly.
How old are these "middle grade students" in the study? This kind of age? I can't see it in TFA.