When Larry Walters was a teenager, he (see) some balloons while he (do) the shopping one day. (Use Past Simple and Past Continuous)

When Larry Walters was a teenager, he saw some balloons while he was doing the shopping one day.

Explanation:

• saw — Past Simple, because seeing the balloons was a short, completed action.

• was doing — Past Continuous, because shopping was an action in progress when he saw the balloons.