Lesson 2 of 6
Objective: SWBAT draw a model to subtract tens.
I like to start this lesson by playing this game. It's a game where the students are given a number and they have to use tens and ones to build the number.
I use this game as a whole group activity and call on random students to come to the Smartboard and play the game. The game is quick and interactive and keeps them engaged. Sometimes, when playing a game like this I put the students in pairs and have them solve the problem together. I then call on random pairs to come to the board and solve the problem.
This activity really gets them thinking about the relationship of place value and drawing models.
I start by writing the subtraction problem 9-5 on the board/chart paper (also available as a PPT Subtract Tens). I call a student up and have them solve the problem. I ask:
- How did you find the difference? (student will explain how they solved the problem)
- What would be the difference of 9 tens – 5 tens? (Since 9 – 5=4, then 9 tens – 5 tens = 4 tens)
I then use base ten blocks, and have students model 30. Have them take away 20. After modeling the problem, have students draw a picture of 3 tens and cross out 2 tens. I ask them how does this picture represent a subtraction problem?
I read the following problem to the students (I like to have them use their base ten blocks first, then draw a picture to represent the subtraction problem): Sammy has 50 shells. He gives 30 shells to his friend. How many shells does Sammy have now?
- How did you solve the problem? (I put out 5 tens and took 3 tens away.)
- How does your drawing show the subtraction problem? (I drew 5 tens, and crossed out 3 tens, and I have 2 tens left.)
I like to continue using this model and practice a few times, moving away from using models to just drawing a quick picture to represent the subtraction problems.
In this picture, the student is drawing a picture to solve the problem:
(MP1) Students use many approaches to find an answer after they have worked out what the problem means and what question needs to be answered. One approach often used is to create a similar but simpler problem to help determine a way to find the answer.
When students are given a problem such as 30 – 10, better problem solvers will not count out 30 counters, remove 10 counters, and count to find the difference. Instead, they will first work to make sense of the problem. They may find that if they think of 30 as 3 tens and 10 as 1 ten, they can simply use something they already know (3 – 1 = 2) to help them understand that 30 – 10 can be thought of as 3 tens – 1 ten = 2 tens, or 20.
I like to use the Subtract Tens_worksheet.in the resources section for student work.
For those struggling students, I continue to have them use a concrete model to subtract instead of drawing a picture.
To close out this lesson I put the numbers 10, 20, 30, 40 on index cards. I give students an index card and have them pair up with a partner and subtract their cards together. We rotate around a few times to give students time to collaborate with each other.