**Lesson Steps**:

The two numbers have three digits each.

Hundreds digit of 100: 1

Hundreds digit of 200: 2

Tens digit of 100: 0

Tens digit of 200: 0

Ones digit of 100: 0

Ones digit of 200: 0

First, we subtract ones digits.

0 - 0 = 0

Second, we subtract the tens digits.

0 - 0 = 0

Finally, the hundreds digits.

2 - 1 = 1

Combining the hundreds digit, tens digit and ones digit, we get the answer:

100

**Answer**:

100

**Grade**: 4

**Level**: Middle school

**Subject**: Math

**Topic**: Addition