posted by jon .
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.
Let Z be income = Px*X + Py*Y
You could say:
subject to the constraint that X*Y=20
or you could say:
subject to the contstraint that Px*X+Py*Y=20