Debating the Advanced Placement high school program

The New York Times ran a public debate on the Advanced Placement (AP) high school class program. Six specialists were invited to comment on last April's Thomas B. Fordham foundation survey of AP teachers, authored by Ann Duffett and Steve Farkas, which had found that AP class attendance has spiked up in the past years while managing to keep a high academic level. The report indicated that the number of high school students who took one or more AP class had increased from 2002 to 2007 by a remarkable 45%, from 1.1 to 1.6 million.

As usual, the reader's comments are more interesting than the expert opinions. But look at the blackboard picture, pasted from the NYT debate lead page. Quiz: what math detail is wrong?