In previous lessons, students added and subtracted multiples of 10 from other multiples of 10 and added two-digit numbers and tens using connecting cubes in towers of 10 and singles, base-ten drawings, numbers, words, and expressions.

This lesson is optional as students may not need more practice developing fluency with adding or subtracting 10. In the first activity, students learn Stage 2 of the Write Numbers center. In this stage, students count by ten forward or backward and write each number as they count. In the second activity, students notice patterns when adding and subtracting ten from any two-digit number.

Why is a strong understanding of the unit ten necessary in order to add or subtract 10 mentally?