Mary will drive across town to take advantage of a 40 percent off sale on a $40

Question:

Mary will drive across town to take advantage of a 40 percent off sale on a $40 blouse but will not do so to take advantage of a 10 percent off sale on a $1000 stereo. Assuming that her alternative is to pay list price for both products at the department store next to her home, is her behavior rational?