In this report we present preliminary results of a study on the relationship between informal out-of-school and formal in-school mathematics and the ways each can inform the other in the development of new mathematical knowledge, in this case concerning computation in base 12, 24 or 60. This study is based on a teaching experiment involving a sequence of classroom activities in upper elementary school based on the use of cultural artifacts, interactive teaching methods and the introduction of new socio-math norms, in an attempt to create a teaching/learning environment focused on fostering a mindful approach toward realistic mathematical modeling. In this way we wish to create a new tension between school mathematics and everyday-life experience in which cultural artifacts, incorporating mathematics, can play a fundamental role in bringing students' out-of-school reasoning experiences into play. (Contains 3 footnotes and 1 table.) [For complete proceedings, see ED500859.]