We are presented with 400 and 6139. Let's work out the sum.

Step 1: We'll start by adding the digits 0 & 9 in column 1 and get 9.

Step 2: We'll start by adding the digits 0 & 3 in column 2 and get 3.

Step 3: We'll start by adding the digits 4 & 1 in column 3 and get 5.

Step 4: We'll start by adding the digits 0 & 6 in column 4 and get 6.

So, 400 + 6139 = 6539.

