Surname 4
is only attainable when the gradient of the budget constraint curve surpasses or is surpassed by
the gradient of the indifference curve. Under the latter conditions, the consumer can do better by
substituting the amount of good y or good x purchased.
Therefore, for a consumer to reach the optimum point in which the income is used optimally to
satisfy consumption occurs when
MUx/MUy or MRS
xy
= Px/Py
As such, optimality condition of a consumer will be achieved when the marginal utility of
consuming good X relative to the marginal utility of consuming good y equals the ratio of the
proportion of good x to the proportion of good y. Marginal utility is the change in customer
satisfaction or utility that would occur if a consumer changes the quantity of a given product
when the other the product remains constant.
Q3: Suppose the consumer chooses a combination of X and Y where the marginal rate of
substitution of X for Y is equal to 2/3. Should the consumer adjust her choice of
consumption? Explain. If so, should the consumer choose more or less of X and/or Y?
If the consumer chooses a combination of X and Y such that:
MUx/MUy=2/3, then the consumer should not adjust her choice of consumption as he or she
would have already reached the consumer optimum.
As illustrated in question Q2, optimum consumption is achieved when MRS
xy
=Px/Py
In line with question Q1,
Px =2
Py =3
Consequently, Px/Py = 2/3
On the other hand, the marginal rate of substitution (MRS
xy
) =MUx/MUy.