No problem, let's work through this together. We're starting with 5421 and 7090 and adding them all up.

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

Step 2: We'll start by adding the digits 2 & 9 in column 2 and get 11.
We'll write down the last digit 1 and carry the 1 to the next column.

Step 3: We'll start by adding the digits 4 & 0 in column 3 and get 5.

Step 4: We'll start by adding the digits 5 & 7 in column 4 and get 12.
We'll write down the last digit 2 and carry the 1 to the next column.

So, 5421 + 7090 = 12511.

