Specifically, for 12th-grade math, the cut scores are 141/300 for NAEP Basic, 176/300 for NAEP Proficient and 216/300 for NAEP Advanced. https://nces.ed.gov/nationsreportcard/mathematics/achieve.as...

The score is an aggregate over questions testing many different skills, so while getting a low score suggests that a student is less skilled, it doesn't immediately tell you which skills they're bad at in particular. So this is exactly the scenario that 'ninkendo was talking about. If you want to know how many students correctly answered a specific question testing a certain skill, you would need the raw disaggregated data, which I don't think NAGB publishes.

I'd like to add that it's intentional that there are substantial numbers of students in each of the four buckets defined by the three thresholds, since the goal is to track the performance of the overall population, not just a few very bad or exceptionally good students.

I should've clarified it was an example, not that literally that one highly particular thing is what all American students are bad at, or that knowing .75 == 3/4ths == 75% somehow causally affects your future or whatever.