Can someone explain the 12 Days of Christmas puzzle to me please?
When you add up the sum of N numbers, the answers comes out to (N+1)*N/2 So for twelve, you get 13*12/2 = 13*6 = 78. As for why this is so, think of pairing the first and last numbers: 1 + 12 2 + 11 3 + 10 4 + 9 5 + 8 6 + 7 You stop at this point because you have accounted for all the numbers. Note that each sum adds up to 13, which is where the N+1 in the formula comes from. As you work you way up the list, you are done when you have accounted for all the numbers, which happens after you have gone halfway up the list. This is where the N/2 part of the formula comes from. It works just as well when N is odd as it does when N is even.
