Okay, we are given 915 and 9987. Let's add them up step by step.

Step 1: We'll start by adding the digits 5 & 7 in column 1 and get 12.
We'll write down the last digit 2 and carry the 1 to the next column.

Step 2: We'll start by adding the digits 1 & 8 in column 2 and get 10.
We'll write down the last digit 0 and carry the 1 to the next column.

Step 3: We'll start by adding the digits 9 & 9 in column 3 and get 19.
We'll write down the last digit 9 and carry the 1 to the next column.

Step 4: We'll start by adding the digits 0 & 9 in column 4 and get 10.
We'll write down the last digit 0 and carry the 1 to the next column.

So, 915 + 9987 = 10902.

