The purpose of this activity is for students to use the relationship between addition and subtraction to make sense of different counting methods for subtracting three-digit numbers. Students analyze two different methods for a given problem, in which a number is subtracted from a multiple of 100. They consider how thinking about a subtraction expression as an unknown addend equation can be helpful, and they discuss how counting on can be a useful method for subtraction. The number line can support this method, so they have the opportunity to make connections between counting on a number line and adding on or subtracting by place, using equations.

• “Which method do you prefer? Explain.” (I prefer counting on, because I count until I get to the number. When I count on, I can do it in parts, so I can make each part something that is easy for me and get to a ten. The number line helped me. I prefer the equations, because they help me see the parts as they are taken away.)

The purpose of this activity is for students to use their understanding of expanded form, place value, and properties of operations to reason about adding and subtracting by place (MP7). Students analyze different methods and representations that show adding hundreds to hundreds, tens to tens, and ones to ones. Students notice that hundreds, tens, and ones can be added in any order. In the next section, the focus will be on strategies based on place value, and will include composing and decomposing tens and hundreds. In the Activity Synthesis, there are discussions that honor all methods, while connecting each strategy to place value, in preparation for the work of the upcoming lessons.

• Invite 1 or 2 students to share how they used each way.
• “Which way did you prefer? Why?” (I prefer Diego’s way, because I can see how things are grouped and I write fewer equations. I prefer Andre’s way, because it helps me remember to add ones to ones, tens to tens, and hundreds to hundreds.)
• “How was it the same and how was it different when finding the sum versus finding the difference?” (It worked either way. I like Diego’s way of finding the difference because the numbers are lined up by place, so the answer is across the bottom when it is done.)