well, sum means addition, so I'm gonna add together 52 and 20.
52
20 +
--------
I start at the ones, and see that 2 + 0 is 2, so drop a 2 below them
52
20 +
--------
2
now, look at the tens place. we add together what we see in the tens place and put it below. 5+2 is 7, so we put that under 5 and 2.
52
20 +
------
72 we see that our final answer is 72, so the sum of 52 and 20 is 72.
