.

       If you rewrite a subtraction problem as an addition problem — for instance, rewriting you can use the commutative and associative properties of addition, which I discuss earlier in this chapter. Just remember to keep the negative sign attached to the number when you rearrange the problem: .

      Subtracting a negative number

      The last rule you need to know is how to subtract a negative number. For example, suppose you want to calculate . Here’s the up and down rule:

       Subtracting a negative number is the same as adding a positive number — go up on the number line.

      This rule tells you that is the same as , so start at :

       When subtracting negative numbers, you can think of the two negative signs canceling each other out to create a positive.

      Q. Use the number line to add .

      A. 2. On the number line, means which brings you to 2:

      Q. Use the number line to add .

      A. 4. On the number line, means start at 6, down 2, which brings you to 4:

      Q. Use the number line to add .

      A. −7. On the number line, means which brings you to −7:

      Q.

      A. 16. Switch around the two numbers with the signs attached:

      Now you can drop

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