It is true that the issue of treaty interpretation is perplexing and this should be emphasized in your answer. You have to start by discussing critically the various interpretative approaches put forward in international law and then juxtapose them with those adopted under the VCLT. The pertinent articles of the VCLT (Arts 31–3) have been considered as customary by the ICJ (most recently in the FYROM–Greececase, 2011); however, the application of these rules by the Court has not been consistent and this substantiates the argument that there is no fixed mathematical approach to treaty interpretation.