7th Grade Section
WME@Kimpton Middle

Understanding Negative Integers III

Jar and CookiesYou are saving money to buy a cool MP3 player. And you can use integers to keep track of your progress. Say we use blue chips and red chips this way:

  • A blue chip represents 1 (dollar), two blue chips 2, and so on.
  • A red chip represents −1 (dollar spent), two red chips −2, and so on.
  • Putting chips of either color in the piggy bank models additon.
  • Removing chips of either color from the piggy bank models subtraction.

Let's see how this model works.

8 + (−3) means: The piggy bank starts with 8 blue chips, now add 3 red chips.

5 − (−3) means: The piggy bank starts with 5 blue chips, now remove 3 red chips.

This tool helps you visualize how to keep count.

Do 8 + (−3) this way:

  1. First add 8 blue chips in the above grid (the piggy bank).
  2. Now add red chips. How many red chips do you need to put in?
  3. What is the answer? (Hint: how many un-paired blue chips remain?).

Next, do 5 − (−2) this way:

  1. Observe we have 5 blue chips and 3 red-blue pairs in the grid already.
  2. Now remove red chips. How many red chips do you need to remove?
  3. What is the answer for 5 − (−2)?

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