In this situation, the buyer buys more of good X than Y since he is deriving more utility from good X. As he keeps on good buying more units of good X, the MU on good X keeps on falling while the MU on good Y keeps on rising due to Law of diminishing marginal utility effect. In other words, the good which is sacrificed, the ** to have more of it increase,& therefore on every additional unit of goods X the MU on good Y rises. The Process continues till both MUX/PX & MUY/PY are equal to each other.
