Just simply answer the question

 Complete this in about one hour!

Jevons demonstrated that consumers’ demand behavior could be derived from consumer’s utility maximization. In a context of two commodities (X and Y), give a simple and brief proof that a utility-maximizing consumer would exchange the two commodities up to the point where the relative ratio of marginal utility between the two commodities exactly equals their relative price ratio (no numerical calculation is required, just explain the logic and reasoning).