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