Differential games link strategic interactions between agents and optimization concerning time. Past and current actions of each player influence all future strategy sets and pay offs through a transition law. Due to high complexity, it is hard to find a Nash-equilibrium within a differential game and it is even harder to get some results in comparative statics. It is the purpose of the paper to describe an approximation routine for an open-loop Nash equilibrium of a simple differential game in exhaustible resources. Excel is applied as it is a wild spread tool.