Today I want to begin by reviewing the comparing of 2 money amounts with the symbols < > = (which I often refer to as the alligator mouth). In previous lessons I introduced the hungry alligator. He opens his mouth to the larger number - the greater side of the greater than/less than sign. He shuts his mouth when the numbers are equal and get the equal sign. I also want to review the importance of the placement of the zero in writing cents amounts.
I put $2.04 and $2.40 on the board. I ask someone to come up and put in the < > = sign. I ask the class to check and see if what the volunteer has written is correct. We read the equation together. I remind students to look at the structure of the number attending to the hundreds, tens and ones place as they compare the numbers.
I put another example on the board. $ 12.50_____$12.05. We repeat the process.
Now I put the following 3 examples on the board and ask their students to copy them and complete them in their math journals.
$45. 90_____$54.09 $16. 00_____$61.06 $ 34.40_____$3.40
I walk around the room to check to see if students understand the comparisons of money where the zero is holding an important place in the number.
I create several grocery store fliers. One says Big Store and one says Little Store. I post each flier on the Smart Board and I give each student a copy of the fliers so they can manipulate the prices easily. I ask students to find the same item (the brand may be different but the product should be the same) and compare the prices. I ask them to pick one of the five items, write it down, decide which store is cheaper and write whether it is Big Store or Little Store next to the name of the item. I show them the video I made that shows someone walking through the process. I take questions and comments about the process that they just saw. Now I tell them that it is their turn to try to compare 2 objects. They may compare the crackers as I did, as well as 4 other items. This allows for success when starting out. Then students may feel ready to tackle one on their own.
Now students will need to use expanded form to compare the two prices and find out how much cheaper one store is than the other. I want them to use math to model the prices. Because of the 3 digit numbers I ask students to use expanded form with hundreds + tens + ones to display the two numbers. I help them review that hundreds are? (dollars) tens are? (dimes) and ones are/ (pennies). We discuss how to place the larger number above the smaller one and then they can subtract Pennies or ones, dimes or tens and dollars or hundreds. I have dimes, pennies and dollars available for students to use to complete the task with manipulatives. They have used the expanded to borrow from next door (the tens or hundreds) to have enough to subtract. The expanded form model makes it easier for students to see the steps they need to take. Common Core standard MP4 expects that students will be able to model with mathematics, and while I hope that students will do this on their own, at this level, I often suggest the model or models that students can use to solve a problem. I encourage the use of the coins and dollars to help students model and compare the two numbers. ( I do allow students to use other models if they are more proficient with a different model.)
Students repeat the comparison of prices with the other 4 items.
I invite all students to bring their completed "bargain" papers to the rug. We discuss the cheapest grocery store item for each set. I ask for a volunteer to show us how he/she figured out which was the cheapest item and by how much. I ask if anyone did the problem differently and ask at least one additional child to show us how they solved the problem.
I know that some students will use the manipulative pennies, dimes and dollars, some will write the problem as a 3 digit number and subtract using the traditional algorithm, other students will write out hundreds + tens + ones and then subtract each one separately and then find the total. Still other students will use a number grid, number line or tally marks to support their solutions
Children check their own papers. They look at the examples that classmates are presenting and they see if they did the problem the same way (when I ask for different ways this helps students to look at how they did the problem) and if they got the same answer as we just found when we had someone share with us. Children are able to see how they did on the shopping comparison because they are checking their papers as we work together.