Suppose Roberta's only consumer choice is to pick the optimal amount of blue pens and black pans. No matter what combination of blue and black pens Roberta is holding, she is always willling to give up one black pens for two blue pens.

a. What is a utility function that describes Roberta's preference?

b. Illustrate an indifference curve that gives every bundle that Roberta trets as being exactly as good as when she has 5 black pens and 5 blue pens.