The swimming pool problem adapted from Algebra in the K-12 Curriculum: Dilemmas and Possibilities, Final Report to the Board of Directors, by the NCTM Algebra Working Group (East Lansing, Mich.: Michigan State University, 1995).
Problem and analysis of algebraic thinking discussed in "Experiences with Patterning," by Joan Ferrini-Mundy, Glenda Lappan, and Elizabeth Phillips, in Teaching Children Mathematics (February 1997), p. 282-288.
Problem B6 taken from Everyday Mathematics, Teacher's Guide to Activities, Kindergarten, developed by the University of Chicago Math Project (New York: SRA/McGraw-Hill, 2001).
McGraw-Hill makes no representations or warranties as to the accuracy of any information contained in the McGraw-Hill Material, including any warranties of merchantability or fitness for a particular purpose. In no event shall McGraw-Hill have any liability to any party for special, incidental, tort, or consequential damages arising out of or in connection with the McGraw-Hill Material, even if McGraw-Hill has been advised of the possibility of such damages.