The pattern of perimeter magic triangles has not yet implemented by teachers for delivering mathematical reasoning. The purpose of this implementation is to describe learning practice for students of Elementary Education Master Program with a mathematics concentration. It is also as a part in mastering mathematics and developing mathematics in elementary school. The study was performed by using qualitative study with 17 students, 13 of them come from outside East Java. The implementation is done in two sessions. The first session is used to introduce and then explore number patterns, and the second session is used to discuss reasoning processes in determining number arrangements in perimeter magic triangles. The second session problem can be classified as a non-routine problem since they never know the problem before, and the way to get the answer needs valid mathematical reasoning. The implementation shows that they gain clarity and in-depth outlook about mathematical reasoning in solving a problem.