What You Need:
- 10 pennies
- 10 nickels
- 3 penny-nickels (3 pennies stacked on 3 nickels)
What You Do:
Now that you've got adding integers covered, it's time to subtract!
- Place 10 pennies and 10 nickels on the table. Explain that the pennies represent positive integers and the nickels represent negative integers. If it helps - think P (pennies) for positive and N (nickels) for negative.
- Write a simple addition problem using integers: -3 + 5=
- Place 5 pennies in a line and below that put 3 nickels in a line.
P P P P P
N N N
- Place the 3 nickels on top of 3 pennies. Explain how positive and negative integers cancel each other out, and ask, “What do you have left?” (Answer: 2 pennies, or positive 2)
- Repeat the process using different addition problems:
(2) + (-6) =
N N N N N N
Place 2 pennies on top of 2 nickels. Ask “What do you have left?” (Answer: 4 nickels, or negative 4)
(-2) + (-6) =
N N N N N N
In this case, point out that there are no pennies (positives) to cancel out the nickels (negatives), so you can just add up the nickels. (Answer: 8 nickels, or negative 8)
- Set up three more nickels with a penny on top of each. (You'll have a total of 13 pennies, 13 nickels on the table.) Emphasize again how positive and negative integers cancel each other out - so these penny-nickel stacks equal zero!
- Subtracting integers is all about changing the signs first. For example:
4 – (-1) becomes 4 + (+1)
Show this with the coins by placing four pennies down, then include one penny-nickel.
Since you're taking away the negative, remove the nickel. What's left is your answer: 5 pennies (positive 5).
One rule to remember is subtracting an integer means you add its opposite!
Other example probelms to work with:
(-4) – (1) becomes (-4) + (-1)
Answer: 5 nickels (-5)
(-4) – (-1) becomes (-4) + (+1)
Answer: 3 nickels (-3)
- Always use the new term “integer” rather than “number”.
- Demonstrate the process 1-2 times, then do the process with your child. Next, have your child solve problems on her own and finally, have your child “teach” the process to someone else in your house!