On the n'th day of Christmas, your true love would give you [n * (n+1)] /2 presents. (This is a simple formula for summing the integers from 1 to n, which you can figure out pretty easily on your own.)
But, to get the TOTAL present count, you would need the summation from 1 to n of [ n * (n+1) ] /2.
It turns out the the TOTAL present count by the n'th day would be [ n * (n+1) * (n+2) ] / 6. (I did NOT figure this one out.)
Plugging 12 into this formula for the 12 days yields: (12*13*14) / 6 = 364, as reported above.
I'm sure you would all be fascinated to know that this same formula can calculate the number of balls in a triangular pyramid in which each edge contains n balls.
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So, if Christmas in VMK is like VMK Halloween, and there ends up being actually,say, 30 days of Christmas, your poor true love will be out 4,960 presents.
Happy Holidays to you all.