We've got two numbers: 90 and 6408. Let's find their sum.

Step 1: We'll start by adding the digits 0 & 8 in column 1 and get 8.

Step 2: We'll start by adding the digits 9 & 0 in column 2 and get 9.

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

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

So, 90 + 6408 = 6498.

