Okay, we are tasked with adding 7121 and 1525. Let's begin!

Step 1: We'll start by adding the digits 1 & 5 in column 1 and get 6.

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

Step 3: We'll start by adding the digits 1 & 5 in column 3 and get 6.

Step 4: We'll start by adding the digits 7 & 1 in column 4 and get 8.

So, 7121 + 1525 = 8646.

