We have numbers 7790 and 9900. Let's start adding them together.

Step 1: We'll start by adding the digits 0 & 0 in column 1 and get 0.

Step 2: We'll start by adding the digits 9 & 0 in column 2 and get 9.

Step 3: We'll start by adding the digits 7 & 9 in column 3 and get 16.
We'll write down the last digit 6 and carry the 1 to the next column.

Step 4: We'll start by adding the digits 7 & 9 in column 4 and get 17.
We'll write down the last digit 7 and carry the 1 to the next column.

So, 7790 + 9900 = 17690.

