@mastersthesis {MS-199112-Sancho-Chavarria,
   type = {Masters Thesis},
   author = {Lilliana Sancho-Chavarria},
   title = {Pertutor: An Intelligent Tutoring System in the Domain of Percentages},
   school = {College of Arts and Sciences, University of Oregon},
   month = 12,
   year = 1991,
   note = {Available at \url{https://www.cs.uoregon.edu/Reports/MS-199112-Sancho-Chavarria.pdf}},
}