Okay, we are given 668 and 31. Let's add them up step by step.

Step 1: We'll start by adding the digits 8 & 1 in column 1 and get 9.

Step 2: We'll start by adding the digits 6 & 3 in column 2 and get 9.

Step 3: We'll start by adding the digits 6 & 0 in column 3 and get 6.

So, 668 + 31 = 699.

