Tins of the brand of Sardines that I always buy are usually $1.75 each. Last week, however, they had been reduced to $1.08 each, so I took advantage of this and bought 48 tins. This week, they are back to $1.75 each, but, to my annoyance, there is a "buy 3 tins, get another 1 free" offer in place. How much more did I pay for the tins I bought last week than if I had bought them this week instead?