[viennagreen]

since you seem to like simulating-- and if you really want to solve this---

i would recommend a brute force guess-and-test, with x,y,z being random INTEGERS between 0 and 50, and a 10% or 20% leeway on the ROI (that is, variables satisfying the equation for a 30 ROI would give a result 29.8 < f(x,y,z) < 30.2 or something like that)...