The purpose of this lesson is to practice the standard algorithm for subtraction and to compare this method to other methods. Students choose a subtraction method, based on the relationship between the numbers in the expression, and explain their choices.

In previous units, students used the standard algorithm to subtract numbers. They interpreted and practiced ways to record 1 larger unit being decomposed and 10 being added to the unit to its right.

In this lesson, students encounter problems where they would need to decompose a series of larger units in order to subtract using the standard algorithm, such as when subtracting a number with non-zero digits from a number with multiple zeros (for example, ). Students consider the merits (the efficiency, likelihood of error, or reliability) of different ways to reason about such differences. They recognize that the standard algorithm may not always be the most efficient strategy for subtracting multi-digit numbers. Students explain how they can use the relationship between the numbers in an expression to select a strategy. 

If students need additional support with the concepts in this lesson, refer back to Unit 4, Section D in the curriculum materials.

What evidence did students give that they draw on their understanding of place value when using the standard algorithm? What evidence did students give that they use their number sense to select a subtraction method?