Due to the six categories of reasoning tests established by Kubinger (2023a) a test concept is suggested which refers to the crystallized facet, concerning numerical contents. Each item presents a mathematical equation with two unknowns and the task is to find values for both of them which fulfill the given equation. Thereby a special multiple choice answer format is used: “2 ´ (1 out of 4)“, that is for each unknown four options are offered. A first draft of such a test (mathematical) Equations with 44 items has been psychometrically analyzed according to the Rasch model. The result shows that five items have to be deleted in order to achieve a-posteriori model conformity. The deleted items are extensively discussed in particular with respect to the given (arrangement of the) distractors. In doing so some unintended variables could be suspected of affecting the items’ difficulties: All above different strategies of processing the task seem to cause the model misfit. Consequently, some means are proposed to prevent a testee from applying the strategy of trial and error by simply checking step by step each combination of the two times four answer options until the solution is obtained.