This paper reports a qualitative research whose subjects were Elementary School Teachers who took part in a workshop about primality of positive integers and the Fundamental Theorem of Arithmetic (FTA). These topics was dealt with from different technological perspectives and analysed under a theoretical proposal connected to the concepts of transparency and opacity of numerical representations and to the "humans-with-media" approach. The interactions occurred in a Post Graduate Program in Mathematics Education and they consisted of two activities created to ask subjects which numbers in a random list would be prime. The analysis showed that participants had difficulties with FTA, which led them to adopt strategies with a high cognitive cost and make mistakes. Likewise, data showed that hindrances were overcome based on the educational proposal planned from a configuration of humans-with-GeoGebra.