OK, let's do this. We've got 1166 and 2521 and we're adding them all together.

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

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

Step 3: We'll start by adding the digits 1 & 5 in column 3 and get 6.

Step 4: We'll start by adding the digits 1 & 2 in column 4 and get 3.

So, 1166 + 2521 = 3687.

