To activate prior knowledge I ask students to respond to the following problems in their math journals.
How much is ten less than 45? Ten less than 88? Ten less than 61? Can anyone tell me what is changing in each of these problems? (The tens place is decreasing by 1 group of 10.)
I ask, "What would 20 less than 45 be?" Again, I ask, "Which place is changing?" (The tens.) We do several more similar problems.
I ask students, "Are adding or subtracting here? How do you know?" (Answers are getting smaller.)
I ask students to come to the rug so we can work at the SmartBoard.
I bring up a picture of a dollar bill, a picture of several objects, and some dimes on the Smart Board. (I gather these from stock Smart Board photos.) If you do not have a Smart board collect a large dollar bill, large dimes and several classroom objects to use. I tell the students that we are shopping today, but only have one dollar to spend. We do not have any change. The dimes belong to the storekeeper. Everything we're shopping for costs less than a dollar. What can we do? I ask for students to make suggestions about how we can use a dollar if something like the ball only costs 80 cents. I am hoping that a child will suggest that I will need change. If no one suggests this I say, "oh dear, I guess I can't buy anything today because I don't want to spend my whole dollar on something that only costs 80 cents. I will have to just go home. Maybe I could just rip the dollar and give the storekeeper a piece. Would that work? These comments help to bring students closer to the idea of change. (Unfortunately in a world where so many people now use credit cards instead of cash, children do not see change being given out on a regular basis, so the idea of making change becomes more difficult.)
I remind students of the lesson when we used a balance scale to pay with a quarter, and had to add pennies or nickels to the scale to make it equal the quarter. I use my hands as the pans of the balance and imitate tipping towards the quarter, with an object and pennies in the other hand.
Today we will think about this with a dollar. I show a toy car and write 80 cents beside it. I bring out the dollar bill. Are the price of the car and the dollar equal amounts? Which is worth more? (The dollar.) What do I need to do to make the two equal? Remember that I have a dollar here and only need 80 cents. I want both hands to be equal to $1.00. The car hand is at 80 and my dollar hand is at $1.00 are they the same? How can I make them the same? Remember I am the store keeper and I have dimes. Could these dimes help me make the two sides equal? How many dimes might I need on the 80 cent side to equal the 100 cent side? ( 2 dimes.) I am modeling here with math (MP4) how we work to find the amount of change that will make the two sides (the object and the dollar equal).
We practice with other objects until the students seem secure with the strategy of counting by tens (dimes) to determine the difference between the dollar and the object being purchased.
I have a deck of cards with 2 of each card. I go around and give everyone a card. I ask them to now find the partner who has the same number and to sit with their partner. Next I give each child 1 dollar, and each set of partners 5 dimes.
I have students draw small pictures of objects that they will price between 50 and 90 cents each. You can also pass out newspaper sales fliers where the prices have been changed to reflect costs between 50 cents and 90 cents in order to save time with this step. I ask students to take turns picking an object, paying for it with a dollar and asking their friend for change. Making Change With Partners of 100. I am asking students to model with mathematics as they use the money and the pictures to practice partners of 10 (MP4).
I observe as the groups are working. If groups show proficiency with this task, I change the prices to between 45 cents and 95 cents, and I give them nickels to add to their coins. They now make change using dimes and nickels.
At the end of the game I ask students to return to put their materials in the math area, and return to their seats. I ask them to take out their math journals.
On the board I draw a picture of a balloon. I put the price of 60 cents on the balloon. I put up a large cardboard dollar bill. I ask students how much I would get back for change if I bought the balloon from them for 60 cents and paid with a dollar. After students have made sense of the problem and solved it (MP1), I ask for a volunteer to tell us the amount of change he/she would give. I ask students if they agree. If they do, we move on to a second problem. If they do not, I ask for a volunteer to come up and count out paper dimes for us to show the change. We count the dimes to find the correct amount.
My second problem I draw a book and put a price of 30 cents on it. I again hold up my paper dollar and repeat the process of having students solve the problem and share their answers.