We've got two numbers: 3157 and 1521. Let's find their sum.

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

Step 2: We'll start by adding the digits 5 & 2 in column 2 and get 7.

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 3 & 1 in column 4 and get 4.

So, 3157 + 1521 = 4678.

