## Thinking Mathematically (6th Edition)

The sum of two three-digit numbers, $105$ and $110$, is a three dig number $215$, and not a four-digit number.
We need to find a counterexample to the statement that the sum of two three-digit numbers is a four-digit number. One such example would be numbers $105$ and $110$. $105 + 110 = 215$ $215$ is not a four-digit number. The sum of two three-digit numbers, $105$ and $110$, is a three dig number $215$, and not a four-digit number.