We are presented with 8521 and 425. Let's work out the sum.

Step 1: We'll start by adding the digits 1 & 5 in column 1 and get 6.

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

Step 3: We'll start by adding the digits 5 & 4 in column 3 and get 9.

Step 4: We'll start by adding the digits 8 & 0 in column 4 and get 8.

So, 8521 + 425 = 8946.

