Marginal rate of substitution is the amount of a good that a
consumer gets equal utility from and thus is willing to give up
inorder to obtain the other good.
If the two goods are perfect substitutes, the consumer will be
Indifferent to the consumption of any good from the two.
Joe views two candy bars and one piece of cake as perfect
substitutes.
Therefore he is indifferent to the consumption of either candy
bars or one piece of cake.
Now let us consucon a utility function U(X,Y). If X is the
amount of goods consumed of a first good then Y is the amount of
good consumed of a second good.
Let X=1( cake) and Y=2(candy bars). Then the MRS chooses the
minimum value among the both as min(X,Y)=min(1,2) = 1