Subtraction with Money

Subtraction with money is often students' introduction to subtraction with decimals.  The rule are exactly the same as subtracting with decimals:

• Line up the decimals before you subtract!
• Remember that whenever a number does not have a decimal point, there's an invisible decimal to the right of the right-most digit.
• If some number have digits behind the decimal (cents) and others don't, add the invisible decimal point, and make sure to put in place holder zeros (when students skip this step, they forget to borrow!).
• Make sure to bring the decimal down into the answer.

Once you make sure you follow the rules above, you just subtract.

Example:

$9 - $6.17 = 

First, set the problem up vertically, lining up the decimal.   When there's no decimal shown, remember, it's right behind the whole number.

$$\eqalign{9&\\-6&.17\\\hline\qquad}$$

It's often easier to fill in the invisible decimal and place holder zeros before lining up.

$$\eqalign{9&.00\\-6&.17\\\hline\qquad}$$

Then, just subtract.

$$\require{cancel}\eqalign{\overset{8}{\cancel{9}}&.\overset{9}{\cancel{\!^10}}\!^1\!0\\-6&.\;1\;\;7\\\hline 2&.\;8\;\;3}$$

Working with money is often a great intro to working with decimals because students often understand, conceptually, the difference between dollars and cents better than they understand the more abstract difference between whole numbers and decimal numbers. Understanding how to line up money problems can be a great, tangible intro to subtracting with decimals.