The Stacked Block Problem

Solution:  There are different ways to organize your thinking -- by row or by column.  There are 4 rows with a sum of 1 + 3 + 6 + 10 = 20.  Or 10 columns with a sum of 1 + 1 + 1 + 1 + 2 + 2 + 2 + 3 +  3 + 4 = 20.