Thu - November 6, 2003

Subtraction is Not Just "Take-Away"



Subtraction doesn't always mean "take-away." No more than "cooking" means "frying."

Use language to introduce and clarify all the different meanings of subtraction. For example:

- I have 3 plates, I want 5, how many more do I need?
- You have $6, if you spend $4, how much will you have left?
- There are 7 people in that line and this one has 3. What is the difference?
- Waiting for the elevator, we're on the 6th floor. If we go down 4 floors, how many more do we have to go?

Subtraction is not just "take away."

- How many more?
~This recipe calls for 3 cups of flour. I've added 2 already. How many more are needed?~
Notice that you are not "taking away" anything in this example. In fact, you are "adding" flour.

-What is the difference?
~Compare the store brand price of $3 with the name-brand price of $4.~
Notice you are not taking away anything. Calling this "4 take away 3" would be confusing.

-Measuring distance.
~You drove 6 miles to a friend's house and you are on your way home and you've gone 2 miles. How many more miles do you have to go?
Again, nothing is being "taken away."

Posted at 01:52 PM     Read More  

©