Question

Suppose a person has a utility function U=YX. What is the minimum level of income required to get a utility of 20? Your answer should be a function of, among other things, Px and Py.

Answer 1

Let Z be income = Px*X + Py*Y
You could say:

Minimize Z=Px*X+Py*Y
subject to the constraint that X*Y=20

or you could say:

Maximize U=X*Y
subject to the contstraint that Px*X+Py*Y=20