Grade: Elementary
Subject: Mathematics

#3244. Subtraction Shopping

Mathematics, level: Elementary
Posted Mon Oct 11 11:20:58 PDT 2004 by Danielle Mattiuz (
University of Pittsburgh at Johnstown, Johnstown, PA, USA
Materials Required: Unifix Cubes, Base ten blocks, overhead projector and overhead projector papers, markers, paper
Activity Time: 15 minutes
Concepts Taught: Two digit subtraction

Academic Standards: 2.1 Numbers, Number Systems and Number Relationships. 2.1.3 A. Count using whole numbers (to 10,000) and by 2's, 3's, 5's, 10's, 25's and 100's. B. Use whole numbers and fractions to represent quantities. L. Demonstrate knowledge of basic facts in four basic operations. 2.2 Computation and Estimation. 2.2.3 A. Apply addition and subtraction to everyday situations using concrete objects. B. Solve single- and double-digit addition and subtraction problems with regrouping in vertical form. 2.5 Mathematical Problem Solving and Communication. 2.5.3 A. Use appropriate problem-solving strategies. C. Select and use an appropriate method, materials and strategy to solve problems, including mental mathematics, paper and pencil and concrete objects.

• Students will be able to subtract two digit numbers.
• Students will be able to check each other's answers for accuracy.

Subject Integration: Writing, Reading

Materials: Unifix Cubes, Base ten blocks, overhead projector and overhead projector papers, markers, paper, colored paper.

Vocabulary: Initials

Instructional Procedures:
• Anticipatory Set: I will be doing a review of two-digit subtraction to prepare for the activity today. Say to the students, "Today we're going to be reviewing our two-digit subtraction today and then we'll be doing a fun activity. First, let's review." Review two-digit subtraction with the students on the overhead. Give examples without borrowing first and then add in a few with borrowing. Ask some students to come up to the front of the classroom to help.

• Developmental activity: Now we will be doing an activity where you get to go shopping and buy what you want from the items we have on sale in the classroom today. You will be working with partners, but I do not want to hear anyone talking unless you are checking each other's answers. If I hear too much talking this will become an individual activity at your seat and I was hoping that we could do this in partners today. I will also be choosing your partners so there is no fighting. The first thing I will be doing is asking you to find an item in the room that you think is worth the amount of money I designate to you. I do not want to hear any whining if someone uses the object that you were planning to use. There are plenty of fun objects in the room to choose from. I will have 17 items, one for each of you and no item will be worth the same amount of money. I will start at $1 and work up in dollar amounts until everyone has had a turn. Once we know how much we want each item to be worth I will ask you to stand up, push in your chair, write down the dollar amount and the object and take a piece of tape from me and tape it where everyone will see where it on the object and then sit back down. Please pay attention to where the papers are being taped so that you know where the object is that you want to buy. I will be keeping a list at the front of the classroom. Now, you will be starting with $99 and you need to finish with $0. I'm going to show you what you need to do now. The very first step is to write $99 on your paper. (Demonstrate all of this on the overhead) Next, you choose an item to buy, go quietly to where it is in the room and write that down on your paper. Since you will be buying it, what will happen to your money? Will you get more or will you lose some? So what is the problem we will be doing? Subtraction or addition? Write your subtraction sign next to the number and draw your line and subtract. You will need to decide if you need to regroup or not. How do you know if you need to regroup or not? Make sure to write down what you are buying next to the number so I know later when I am checking them. I also want your initials next to the problems that you do so that I know you both did your share of the work and so that I can grade the papers. Once you have an answer you will give your paper to your partner so that they can check it and then they take their turn of choosing an object to buy. When they have chosen their object they will write the answer from the last turn and rewrite it below so that that number will be their new starting number for their problem. They will then write the amount of the object that they have chosen underneath that and subtract. You may use the same object more than once. You may use the base ten blocks or the unifix cubes if you need to. This will continue, switching turns and checking each other's answers until you reach $0. It is possible. If you end up with $3, there will be an item worth $3 that you can buy to make your total equal $0. So, what are you going to do? Are you going to talk? Are there any questions? Make sure you are careful when you do this because I will be collecting at the end of class for a grade. If you get finished early you have some work from earlier to do. Alright, I will be going around the room by rows to decide upon our objects so you will not need to raise your hands to be picked upon. ________, will you please choose our $1 item? Please come up and write it down on this piece of paper nice and big, writing the money amount first. Don't forget about the $. Hang the paper up on the item if you can, if not, next to it. Thank you. (Continue like this, increasing money increments until all of the students are done). I will ask the students again what they will be doing, and then I will tell them who will be partnered with whom. I will tell them to stand up, without talking, find their partner and then go quietly around the room and start shopping. Circulate the room to answer questions and make sure the children are working correctly and efficiently.

• Closure: Ask the students to remove the paper from the object that they hung up, not anyone else's, just their own and bring it back to their seats. Ask them if they had a hard time with this or if it was easy. Ask them how they decided to go about choosing items and why they chose the items that they did. I will continue with this conversation until the students have said all that they wanted to about the topic or until we run out of time.

• Assess the accuracy of students' work.
• Assess the students' ability to work in groups.

Special Needs Adaptations: A student with ADHD is in my classroom with an aid. He is usually well-behaved with the aid near, so I will place him in a group with a well-mannered child and he should be fine. If he acts up the aid takes care of him. If he becomes too disruptive I will remove him from his group.

Extension/Enrichment: I could give the students a page in their workbooks to do for homework if I see that they are having difficulty with the subtraction.

Technology Integration:
• One computer: I will use the computer at the beginning of the classroom on a power point presentation. It will include 6 slides each describing a different step in the activity which we will be doing. For example one slide will depict a math problem of $99 - $10 with the answer not shown. The last slide is a complete list of rules that the students can refer to at any point during the exercise.
• Six computers: When the children finish the activity they will be allowed to go to the computers in the back and play baseball on They must choose subtraction and in the box labeled "difficulty" they are to choose medium. They may move on to hard if they are doing the problems with little to no difficulty.
• One computer for each child: Students will either use a calculator available to them on their computer or on this website:,1871,21904-175851-24-14522,00.html. They will type in 99 and subtract all of their shopping amounts using this calculator. It should be similar to 99-11=88, 88-5=83, 83-10=73, and so on. This will be used for self correction on the students' behalf.