School math books, nonsense, and the National Science Foundation

Problem: Find the slope and y-intercept of the equation $10=x−2.5$⁠.

Solution: The equation $10=x−2.5$ is a specific case of the equation $y=x−2.5$⁠, which has a slope of 1 and a $y$-intercept of $−2.5$⁠.

This problem comes from a 7th grade math quiz that accompanies a widely used textbook series for grades 6 to 8 called Connected Mathematics Program or CMP.¹ The solution appears in the CMP Teacher’s Guide and is supported by a discussion of sample student work.

Richard Askey, a mathematician at the University of Wisconsin at Madison, reported, “I was told about this problem by a parent whose child took this quiz. The marking was exactly as in the text.”² Students instructed and graded in this way learn incorrect mathematics, and teachers who know better...