Solve for the Missing Addend: ? + 900 = 900

To solve this problem, we'll apply the basic property of addition involving zero:

We start with the equation given in the problem:

• Step 1: To find the unknown number $x$, observe that adding 900 to an unknown number results in 900.
• Step 2: Subtract 900 from both sides to isolate $x$:
  $x + 900 - 900 = 900 - 900$
• Step 3: Simplifying gives:
  $x = 0$
• Step 4: Thus, the missing number is 0, as adding 0 to any number retains the original number due to the identity property of addition.

Therefore, the solution to the problem is $x = 0$.