Modelling decisions in organisations is a challenging task. Deciding which modelling language to use for the problem at hand is a fundamental question. We investigate the Decision Model and Notation (DMN) standard and the IDP knowledge base system (KBS) in their effectiveness to model and solve specific real-life case problems. This pa- per presents two cases that are solved with DMN and IDP: (1) Income taxation for foreign artists temporarily working in Belgium; and (2) Reg- istration duties when purchasing real-estate in Belgium. The solutions in both DMN and IDP are examined and restrictions and opportunities of both methods are elaborated upon for each type of problem presented in the case studies. Additionally, compatibilities and synergies between DMN and IDP are identified