Continuing to build mental math skills

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Objective

SWBAT subtract ten from a two-digit number mentally.

Big Idea

It is a difficult task for first graders to begin using mental math. This lesson is part of a series of lessons to give them practice with subtracting ten from any given two-digit number.

Rev Them Up

5 minutes

I will review with my class on how to subtract using base ten blocks. It will be important for today's lesson that they know how to use base ten blocks correctly because it will serve as a concrete basis for them to subtract ten (1.NBT.C.6). First graders begin working on tens in Kindergarten. This lesson will build on that knowledge as they use mental math to subtract 10.

I will have the following discussion with them:

  • Students, what happens as I take a base ten away? (the number gets smaller)
  • What does smaller mean? Can anyone give me an example? (23 is smaller than 54, smaller means less)

Each time I pose the questions, and I will ask them to turn to their group and discuss their answer with their group. I will ask them to pick a representative to share the groups answer with the class for each question.

The focus of today's lesson is not to continuously take away base ten blocks to subtract ten.  I want them to begin mentally subtracting the ten and noticing that the tens place in a two-digit number is decreasing by one (1.NBT.C.6). I am using this intro to review what it is that we are doing, using the concrete manipulative of the base ten blocks.

Whole Group Interaction

10 minutes

First, we will use our class number line as a concrete model to solve several minus ten equations. I want to use this concrete method to continue building a foundation to think abstractly and quantitatively to eventually subtract ten mentally (MP2). I will have them help me with 42 minus ten and 33 minus ten; I will point and jump while they subtract ten with me aloud. We will all be doing this together aloud.

Second, I will transition to how minus ten looks on a 120 chart. I must show them the connection between the two tools. Counting on a number line is easy to see because it flows. A 120 chart requires you to finish a line and then go to the beginning of the next line. Seeing it both ways is beneficial.  Students are assisting me in this discussion to see which way they move to subtract. We are not yet using mental math, but it is worthy to use because it is a tool that can support your struggling students and help more advanced students review the underlying place value concepts that help make these equations work. This student is using the chart to identify his answer. Students can identify plus one, minus one, plus ten, and minus ten on 120 charts. I will begin on 66, subtract ten by pointing and counting until I reach 56. I will repeat again with 89 minus ten and 23 minus ten.

Then I will ask: Do you see a pattern? Where did we end up every time we subtracted ten? (above the number we started with)

I will draw from my name bucket to select students to answer questions about place value to make sure everyone gets a chance as we go through our discussion. My ultimate goal is for them to identify that minus ten means to decrease the tens place by one, in a two-digit number. Students look at the chart and make the connection that if they go up a column, it will decrease by ten.

I will do another example and start by coloring 104 on the 120 chart. Then I will subtract ten by pointing and sliding my finger ten spaces with my students counting aloud; I will land on 94 and color 94. I will continue this method until we reach 4. Last, I will go back and highlight the number in the tens place and ask:

Do you see something happen up this column or a pattern? (the tens place is decreasing by 1 as you go up)

Independent Practice

10 minutes

My students would much rather play a game than to do a practice sheet, and I found the perfect game for today's lesson. You can go here to print the 10 less game sheet and spinner. Using a 120 chart for practice will reinforce the idea that minus ten requires students to go up a column on a 120 chart because each square going up is decreasing by 10. Instilling this concept in their thought process will add to their ability to eventually solve mentally. You can select the starting number for the game. Watch this game video to see how the game works.

Closing

5 minutes

I will pass out our dry erase boards and summarize our lesson by having them answer some equations. I will call out a minus ten from a 2-digit number problem. They will write their answer on their board and then hold it in the air to show me their answer.