OK, let's do this. We've got 2121 and 5454 and we're adding them all together.

Step 1: We'll start by adding the digits 1 & 4 in column 1 and get 5.

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 1 & 4 in column 3 and get 5.

Step 4: We'll start by adding the digits 2 & 5 in column 4 and get 7.

So, 2121 + 5454 = 7575.

