No problem, let's work through this together. We're starting with 420 and 456 and adding them all up.

Step 1: We'll start by adding the digits 0 & 6 in column 1 and get 6.

Step 2: We'll start by adding the digits 2 & 5 in column 2 and get 7.

Step 3: We'll start by adding the digits 4 & 4 in column 3 and get 8.

So, 420 + 456 = 876.

