Let's get this math done. We have 9120 and 879 and we're going to add them all together.

Step 1: We'll start by adding the digits 0 & 9 in column 1 and get 9.

Step 2: We'll start by adding the digits 2 & 7 in column 2 and get 9.

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

Step 4: We'll start by adding the digits 9 & 0 in column 4 and get 9.

So, 9120 + 879 = 9999.

