Alright, let's solve this problem step by step. We have 3121 and 1605 and we're adding them together.

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 & 0 in column 2 and get 2.

Step 3: We'll start by adding the digits 1 & 6 in column 3 and get 7.

Step 4: We'll start by adding the digits 3 & 1 in column 4 and get 4.

So, 3121 + 1605 = 4726.

