Previously, students learned to record subtraction, using an algorithm in which the numbers are written in expanded form. They made connections between the structure and the steps of the algorithm and those of the base-ten diagram that represents the same subtraction. In this lesson, students take a closer look at the algorithm and use it to find differences. Students also examine a common error in subtracting numbers when decomposition of a place-value unit is required. When students discuss shown work, they construct viable arguments and critique the reasoning of others (MP3).

Today students analyzed a common error when subtracting within 1,000, before they used an algorithm to subtract. How did the analysis of the error affect their work with subtracting within 1,000?