Let's solve this addition problem. We have 7866 and 6915, and we need to add them together.

Step 1: We'll start by adding the digits 6 & 5 in column 1 and get 11.
We'll write down the last digit 1 and carry the 1 to the next column.

Step 2: We'll start by adding the digits 6 & 1 in column 2 and get 8.

Step 3: We'll start by adding the digits 8 & 9 in column 3 and get 17.
We'll write down the last digit 7 and carry the 1 to the next column.

Step 4: We'll start by adding the digits 7 & 6 in column 4 and get 14.
We'll write down the last digit 4 and carry the 1 to the next column.

So, 7866 + 6915 = 14781.

