10 more or less on a 120 chart

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SWBAT mentally find 10 more or 10 less than a given two-digit number.

Big Idea

Before First Graders are able to solve mentally, they must develop a concrete understanding of 10 less or 10 more than a given two-digit number. This lesson provides my students an opportunity to transition to solving mentally.

Rev Them Up

2 minutes

I like to start my math lessons out on a positive note and get my kiddos excited about numbers. I tell my kids that math is another language and each day we are adding to our vocabulary. My students love to play a game called "Around the world," and it is applicable to most topics.

I began today's lesson with addition practice using the game format. I will pick two students to stand up next to each other and then I will verbally state a problem that relates to today's objective, ex. 23+10.  Whoever says the answer first, wins. If there is a tie, I will give another problem until there is a winner. The winner will be matched with another person and the game will continue. I go all the way around the room until everyone has had a turn. This will give me a chance to see who is already doing mental math. 

Whole Group Interaction

15 minutes

Many of my students continue to struggle with being able to mentally add 10 to a two-digit number. Today I will use a 120 chart as a way to support my students in finding plus or minus ten. This is not the first lesson I have taught my students on this concept, but it is a Common Core standard that requires lots of practice.  First graders must transition from solving these types of problems with manipulatives or lengthy strategies to being able to use the structure of our base ten place value system to solve these problems mentally (MP7).  Being able to solve mentally is important for students to later be able to solve more complex problems (1.NBT.C.5). I chose a 120 chart for today's lesson for my students to see how ten plus and ten minus moves them above or below their beginning number. It would also be appropriate to use number lines and ten frames as concrete tools for students to see movement from decade to decade or off the decade numbers if you'd prefer to work with those tools instead. 

First, I pulled up a 120 chart on the Smart Board and began asking for volunteers to help me solve different problems:

  • What would 62 plus 10 be? How can we solve it using the chart? (You can watch the chart video to see how my student responded) I will be pointing out the column and counting 62, 72, 82 ... each time we add ten, we are just changing the tens digit by 1.
  • What is 54 minus 10? How can we figure out the answer on our chart? (We can put our finger on 54 and slide it up one row.) I will be pointing out the column and counting 54, 44, 34... each time we subtract ten, we are just changing the tens digit by 1.

I want them to transition from using concrete tools to solving mentally. A pattern can be identified in adding or subtracting tens by using a 120 chart, and pointing out this pattern will help them solve mentally because they can recall the counting pattern of the numbers. 

It is okay to start with volunteers and use those students who feel confident with the skill to gain answers to your questions. There are some lessons that I want to talk with every student in the class and give them an opportunity to solve a problem during our whole group interaction. This allows me to identify what everyone understands. I will continue and ask every student to come and help me solve a problem using the chart in this lesson because it is not a new concept and many students will be able to respond quickly. 

Independent Practice

10 minutes

I will print this 120 chart  and copy for students to use. A few of my students are already solving mentally and will not need a chart.  I will print this worksheet for my students to use for independent practice.  They have developed a good understanding of the quantities that the numbers represent and what steps make sense to them to solve the problems, so the 120 chart is a tool that is only necessary for those that are not ready to solve mentally but are building the pathways to making that leap. If students have not noticed the counting pattern that occurs in each column, use this time to point it out to your students who are not solving mentally yet. 

Example: If the student is solving 46 minus 10, point at the 46 on the chart and count backwards by ten 36, 26, 16. Point out the pattern deliberately and discuss why this pattern is occurring (we are taking away a ten each time - show using base tens if necessary); then begin guiding them in finding the pattern themselves as he or she is solving. You could even take a moment and have the whole class count by tens from any starting number and count backwards by ten from any starting number. You need to let them know that counting on the decade numbers is not the only way to count by tens.


5 minutes

I will ask my students to turn to their neighbor and describe what they do to solve a problem that asks them to add or subtract 10 from a 2-digit number. I will walk around the room and listen to their conversations.