On the first day of Christmas, my true love gave to me… some mildly interesting combinatorics! **The Twelve Days of Christmas** is a Christmas favourite, and whether you interpret the lyrics to mean that each day only one type of present is given, or more literally to mean that on each day you are given all the previous presents again, the question of how many gifts are given in total has been a classic festive brain-teaser for as long as we’ve had summation formulae.

**Think Maths **have cleverly put together a pack of school resources which includes worksheets on calculating the total number of presents for different key stages, and some activities around the **Christmas Price Index** (XPI) – the seasonal measure of retail prices, calculated by adding up the cost of all the gifts mentioned in the song. There’s also a chance to analyse past XPI data and make a graph out of tinsel.

*This post is part of the Aperiodical’s 2018 Aperiodvent Calendar.*

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