Okay, we are given 840 and 151. Let's add them up step by step.

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

Step 2: We'll start by adding the digits 4 & 5 in column 2 and get 9.

Step 3: We'll start by adding the digits 8 & 1 in column 3 and get 9.

So, 840 + 151 = 991.

