Each day they recite what was gifted to them. Each day they get all the gifts of the days before plus a new gift. So, for example, the first day you get a partridge. But on the second day you get two turtle doves and a partridge. So now you have 3 partridges and two turtle doves. Repeat as the days go up. So the total number of any given item will be equal to the number given times the result of 13 minus the day it was given, or Total = N(13-d). So golden rings is 5(13-5) which is 40. Partridges is 1(13-1) which equals 12. Etc etc.