When we add numbers together they look like this:
We put the biggest number at the top and add from top to bottom.
First, we add the numbers on the right in the units column: 2 + 1 = 3
Then, we add the numbers on the left in the tens column: 10 + 0 = 10
Together that makes 13.
12 + 1 = 13
Here's another example:
First, we add the numbers on the right in the units column: 0 + 6 = 6
Then, we add the numbers on the left in the tens column: 10 + 0 = 10
Together that makes 16.
10 + 6 = 16