OK, let's do this. We've got 2501 and 400 and we're adding them all together.

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 0 & 0 in column 2 and get 0.

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 2 & 0 in column 4 and get 2.

So, 2501 + 400 = 2901.

