Add two 3-digit numbers

Up to two times — once from the ones to the tens column and again from the tens to the hundreds column — so students need to check both places for regrouping opportunities.

Forgetting to add the carried 1 to the next column is the most frequent mistake — encouraging students to write the carried digit clearly above the column helps prevent this.

The regrouping process is identical regardless of the number of digits, so mastery here transfers directly to adding 4-digit numbers — students simply apply the same steps to one additional column.