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Monday, April 16, 2007

Her students don't know how to work with percents!

Rudbeckia Hirta reports that her college students don’t know how to work with percents. I would like to say I’m surprised. I would like to say that all Weston High School students know how to work with percents.

I would like to say that.

But I would be lying. I know perfectly well that there are plenty of Weston High Schools who don’t know how to do certain problems involving percents — at least the ones like “7 is 6% of what number?” But surely they can do some of the problems that some of Hirta’s college students can’t do:
We did a problem where they were told the price of the item ($20), the sales tax rate (8%), and the percent discount (25%). The question asked whether getting a discount of 25% and then not having to pay the 8% tax could be thought of as a 33% savings from the usual, non-discounted, with-tax price. (This problem was based on an advertisement that I got in the mail claiming just that.) One student couldn’t figure out why the $20 item was $21.60 after tax. No one could decide whether or not paying $15 for the item would be a 33% discount from the $21.60 normal (with-tax) price.

We did another problem in which we were calculating what percent of the positive tests from a medical test were patients who actually had the condition. We found that of the 604 patients who tested positive that only 10 of them had the condition (the remaining 594 were false positives). I calculated the rate as 10/604 and stated that it was about 1.7%. One of my students raised her hand and asked why 10/604 is about 1.7%.

In another problem, we were trying to calculate 60% of 495,000. A lot of students in my class didn't know how to do it.
All right, maybe I’m just too optimistic. But I really do believe that almost all Weston High School students could calculate 60% of 495,000 — especially with the use of calculators, which these college students also had. In fact, I’m pretty confident that almost all Weston Middle School students could do these problems as well.

Hirta also points out that her “students prefer to study the computational tasks that they're used to but can not do (and would fail if we spent significant time on) but despise working with the abstract concepts that they are smart enough to do a reasonable job of understanding.” That doesn’t surprise me either, but that’s an issue for another day.

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