Yesterday I was out shopping with my daughter and we got to discussing a price tag that said "Two for the Price of One". She asked "What would it cost if you wanted 3?" and I explained that you could work it out by splitting the 3 into 2 + 1. The "Two for One" would only apply to the 2, and you would have to pay the full price for the 1. I then summed it up as "Half Price for Even Quantities". Thinking about this now, a better way to sum it up would be as (p div 2) + (p mod 2), where p is the original price in pennies.