Okay, we are tasked with adding 1591 and 1670. Let's begin!

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 9 & 7 in column 2 and get 16.
We'll write down the last digit 6 and carry the 1 to the next column.

Step 3: We'll start by adding the digits 5 & 6 in column 3 and get 12.
We'll write down the last digit 2 and carry the 1 to the next column.

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

So, 1591 + 1670 = 3261.

