Let's solve this addition problem. We have 6278 and 2421, and we need to add them together.

Step 1: We'll start by adding the digits 8 & 1 in column 1 and get 9.

Step 2: We'll start by adding the digits 7 & 2 in column 2 and get 9.

Step 3: We'll start by adding the digits 2 & 4 in column 3 and get 6.

Step 4: We'll start by adding the digits 6 & 2 in column 4 and get 8.

So, 6278 + 2421 = 8699.

