The advertisement constituted an offer to the whole world. Sue had effectively accepted the offer, like Mrs Carhill, and Gus would be obligated to give the discount. Admittedly this question is a bit vague. This question is on Consideration. Past consideration is where the act or forbearance which is alleged to constitute the consideration occurred before the promise.
Consideration is the price paid for the promisor's promise.
Logically speaking, therefore, the act or forbearance that is alleged to constitute the consideration must occur as a response to the promise. Consequently the promise must have been in existence and must have been communicated to the promisee before any act or forbearance which is alleged to constitute the consideration could occur. In other words if what is done is not done as a reaction to the promise it cannot constitute good consideration. A classic case in point here is Eastwood v. Kenyon . In the question Walter's promise to give Joe a lottery ticket was made after Joe had towed his car to the garage.