The lowest quartile presumably would never have been able to master many of these rules in the first place. Some of the information resembles the stuff that a friend of mine experienced when he went in to do tutoring for disadvantaged students in Boston when he was getting his doctorate at MIT. At first my friend was totally taken aback at the level of ignorance (e.g., the inability to see the relationship between 1/10 and 10/100).

Again, from my experience, most can master it, given some 100x the effort and time investment by both teacher and student, compared to those somewhat above average. It took one of my students about 3 years and countless problems to get comfortable with percents (e.g. to calculate sale price). The skill seemed there for a short time after learning, then dissipated after a while, reverting to the old "I don't understand percents!" mental block.

Whether such an investment is worthwhile is a different story.