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A Pencil and a Sticker


Alignments to Content Standards: 2.OA.A.1

Task

A pencil costs 59 cents, and a sticker costs 20 cents less. How much do a pencil and a sticker cost together?

Solution

The pencil costs 59 cents, and the sticker costs 20 cents less than that:

Bar_diagram_f65a38129f72884335e771a7f043b686

So the sticker costs 59-20 = 39 cents.

The cost of the two together:

Bar_diagram_2_62fa352c557c7198393b2d7434970769

is 59 + 39 = 98 cents.

Trent Reynolds says:

about 4 years

I have been working with teachers a lot lately with this type of modeling and it is a great way of representing the situation and illuminating some paths for finding the solution. It does not force students to use subtraction, for example some may choose to count on from 20, use tree diagrams or even partial differences. No matter the method, the model supports all of them.

Shelbi says:

about 4 years

I was talking to some teachers about this problem today and there was some confusion about the "solution" being shown only as a bar diagram given that the question didn't require that representation. I wonder if there is a way to more clearly label what is intended by the solution. Is it to provide information about how students might represent the problem or how teachers might support students in understanding the problem better? Maybe a commentary is warranted like on some of the other items?

shante says:

about 4 years

Thinking blocks can walk you through why you need that number as for the representation. I like to play with those with my classroom.

Ms Hodge says:

over 5 years

Looks a lot like Singapore Math's Model Drawing method. My 2nd graders have connected very well with this problem solving method this year.

Kristin says:

over 5 years

These kinds of diagrams are used in Singapore Math and in other places as well. In the Common Core Standards, they are called "tape diagrams" and are named that explicitly in 6th grade. Students should also see area models and number lines in addition to this particular kind of length model.