Solve the Equation: Finding the Missing Number in 1000 + □ = 1100

To solve the equation $1000 + \Box = 1100$, we need to determine the value of $\Box$ that makes the equation true.

• Step 1: Recognize that the equation has the format $a + b = c$.
• Step 2: Identify the known values:
  - $a = 1000$
  - $c = 1100$
• Step 3: Rearrange the equation to solve for the missing number $\Box$:
  $\Box = c - a = 1100 - 1000$
• Step 4: Perform the calculation:
  $1100 - 1000 = 100$

Therefore, the missing number $\Box$ is $100$.

Selecting from the given choices:

• Choice 1: $1$ - Incorrect.
• Choice 2: $10$ - Incorrect.
• Choice 3: $100$ - Correct.
• Choice 4: $1000$ - Incorrect.

Thus, the correct choice is Choice 3: $100$.