Subtraction - Reverse Addition answers questions


Number line
Number line

Subtracting numbers is just the same as adding them, only walking the other way along the number line. To subtract 3 from 8, you start on the number line at 8, and walk backward three steps until you get to 5 - that's the answer: 8 - 3 = 5. Subtraction tells you how many cookies you would have left, if you started with eight cookies and then you gave three of them to your best friend.

You can also think of subtracting numbers as being the same as adding a negative number. If you added 8 + (-3), you would also get 5. Why is this true? Because 8 + (-3) is the same thing as -3 + 8, because addition is commutative. And -3 + 8 means that you should start on the number line at -3, and walk forward eight steps until you get to 5, and that's the answer.

A subtraction game with jumping
How to subtract big numbers (borrowing).

Bibliography and further reading about subtraction:

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Professor Carr

Karen Eva Carr, PhD.
Assoc. Professor Emerita, History
Portland State University

Professor Carr holds a B.A. with high honors from Cornell University in classics and archaeology, and her M.A. and PhD. from the University of Michigan in Classical Art and Archaeology. She has excavated in Scotland, Cyprus, Greece, Israel, and Tunisia, and she has been teaching history to university students for a very long time.

Professor Carr's PSU page

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