Rudolf Rational obeys the weak axiom of revealed preferences.His preferences don't change over time.One year he could afford bundle x but bought bundle y.If another year he buys bundle x, then he can't afford bundle y.

Rudolf Rational's behavior follows the weak axiom of revealed preferences, which means that if he chooses bundle y over bundle x in one period, it must mean that y is revealed to be at least as preferred as x. However, if in the next period he chooses bundle x over y, it must mean that x is now revealed to be at least as preferred as y. This is consistent with Rudolf's preferences not changing over time. Therefore, if he could afford x in one year and bought y instead, it implies that y was revealed to be at least as preferred as x in that year. If in another year he buys x instead of y, it must mean that x is now revealed to be at least as preferred as y and he can no longer afford y. Hence, the statement "If another year he buys bundle x, then he can't afford bundle y" is true.