Adding a 1-Digit Number to a 2-Digit Number (by counting on)

We can find the sum of a given one-digit and a two-digit number by counting on.

For example: Let's find the sum of 15 and 4.

In order to find the sum, we need to count up.
 

15 + 4 = ?
 

Let’s start counting from 15 and count up 4:
 

15, 16, 17, 18, 19
 

Now,
 

15 + 4 = 19
 

So, the sum of the given numbers is 19.