Pile up a pile of steel pipes, 11 at the bottom, one less at each layer upward, until one. How many steel pipes are there in this pile?

Pile up a pile of steel pipes, 11 at the bottom, one less at each layer upward, until one. How many steel pipes are there in this pile?

11-1 + 1 = 12 floors
There are: (11 + 1) × 12 △ 2 = 72