Building mental math ability

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SWBAT add ten to 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 at adding ten to any given two-digit number.

Rev Them Up

5 minutes

I will review with my class how to add using a number line. It will be important for today's lesson that they know how to use a number line correctly because it will serve as a concrete method for them to add ten (1.NBT.C.5). First graders begin working on tens in Kindergarten. This lesson will build on that knowledge as they use mental math to add 10. I will take my class over to our class number line; it is stretched across our library counter from left to right. You can go here to access a free printable number line.

I will have the following discussion with them:

  • Students what happens to the numbers as I point and jump to the right? (they get bigger)
  • What does bigger mean? Can anyone give me an example? (they are worth more; 23 is greater than 15; bigger means more)
  • Which direction should I jump if I'm going to add? (to the right) Why? (Because numbers increase when we go that direction on this number line.)

After each question I will ask my students to turn and share their answer with their neighbor before I pick someone to answer. I am using this time period to just review the number line. From here we can begin working towards the goal, which is for them to begin mentally adding ten and noticing that the tens place in a two-digit number is increasing by one each time we add ten (1.NBT.C.5). 

Whole Group Interaction

10 minutes

First, we will use our class number line as a concrete model to solve several plus ten equations. I want to use this concrete method to build a foundation for thinking abstractly and quantitatively to add ten mentally (MP2). I will have them help me with 42 plus ten and 33 plus ten to start; I will point and make jumps with my pointer while my whole class adds ten with me aloud.

Second, I will transition to how this plus ten looks on a 120 chart. I must show them the connection between the two tools because we have worked with the 120 chart more in previous lessons and I want them to make the connection between what we are doing here and what we did in those lessons.

Third, my ultimate goal is for them to understand the reasoning behind increasing the tens place in a two-digit number by one when adding ten.

We do a few more examples as a group with the number line, and I follow up each example by asking student to pair share with questions like:

  • What digit changed when we added 10 to the first number? (the tens place digit)
  • How much did it change by? (1) How much did its value change by? (10)
  • What did you notice about the ones place? (it didn't change)

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 more game" sheet and spinner.

I always pay attention during my whole group interaction activities and identify students who provide incorrect answers or offer blank stares. These are the students that I will pull together with me and provide with more guidance in the independent activity before I release them to work on their own. After I have all of my students working, I will walk the room and check for any misconceptions. Since there are multiple choices on the spinner, I will be checking to see that everyone is going the correct direction for plus and minus and identify if they are paying attention to if they spun one or ten to be added or subtracted. Multiple choices sometimes throw first graders off, and I want to support them and help prevent mistakes. 


5 minutes

I want to do a quick check to see who may already be solving plus ten mentally. My students love to play around the world. I use it throughout the year and just change the concept. Here you will see the game in action as we played it with subtraction.

Here's how to play: I will pick two students to stand up next to each other and then I will verbally state a problem, 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.