SWBAT use division and repeated subtraction strategies to determine the cost of snack items purchased in bulk.

Real world application and real world items make complex tasks engaging!

9 minutes

Prior to releasing the students to work with the bulk items on their own, I review the concept of profit because it is very new and not something most of them have ever thought about.

I pick a piece of fruit (apple, banana, orange) and ask the students what it costs. An approximate cost for any of these items is $1.00. Then ask the students how much money the store makes from the sale of this item. Their answer will usually be $1.00. In order to keep it reasonably clear, I don’t talk about store overhead or salaries. I do teach them that the store buys the fruit from a supplier, and I give them an estimated cost, perhaps 25 cents. Then I ask them, if the store had to buy the fruit for 25 cents, and sells it for $1.00, how much money has the store actually earned?

40 minutes

Today students work with the actual fundraiser items.

I divide students into a number of groups that matches the number of items. In my classroom, this ends up being six groups. Each group starts out with an item that will be sold at the fundraiser. I bring the items to their table or their spot on their floor and then task them with finding the cost per item and then a reasonable selling price and the profit per item. At the conclusion they are responsible for writing it out and being able to explain their answer but their route to a solution is up to them. Some draw models on their white boards, some write out equations, and almost all of them move the items around into groups as they are thinking the problem through.

Here a student is talking through the first steps of calculating price per item. You can see on his paper that he is starting to do repeated addition or subtraction as a possible route to per item price.

In this group children are simultaneously using different strategies to obtain their cost per item.

This student has chosen to go back to paper and draw out an array diagram to try and work out the cost per item.

This group uses a decomposed adding on strategy (tens, tens, then ones or twos) to arrive at the item cost of 22 cents per bag of chips.

After they figure the cost per item then the next step is to figure out how to make a profit. They calculate the total profit and the profit per item.

7 minutes

To conclude the lesson, each group needs to explain to the class what their cost per item is, how they calculated it, what they will sell each item for and what the profit will be, either per item or for the whole unit of items.