If you were in the middle of the desert,came upon a lemonade stand,and paid $20 for a glass of lemonade,

A) you would definitely be overpaying.
B) you would have gotten at least $20 of utility from the lemonade.
C) there is no way to determine whether or not you overpaid.

The concept of utility in economics suggests that if you willingly pay $20 for a glass of lemonade, especially in a situation like being in the middle of the desert, it indicates that the satisfaction or utility you expect to gain from the lemonade is at least worth $20 to you at that moment.