Ortiz, Maria, and Tomas Ortega. “DESIGN OF AN INTERVENTION FOR THE TEACHING-LEARNING OF MENTAL CALCULATION IN THE CLASSROOM”. Revista Latinoamericana de Investigación en Matemática Educativa, vol. 5, no. 3, Nov. 2002, pp. 271-92, https://www.relime.org/index.php/relime/article/view/557.