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Decimal Addition

Add and subtract decimals by lining up the decimal points.

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Check Your Shopping Charges

Shopping can be fun, but the more you buy, the more you need to keep an eye on what you spend. Not only is it complicated to count and remember numbers in dollars and cents, but it?s also too easy to forget those numbers, especially when you want to focus on more shopping.

Amazing But True

If you look at a list of prices, you will find that the cents vary over all the numbers from 0 to 99?no higher than 99 because 100 cents is another dollar. This fact allows us to round prices to the nearest dollar for easier addition, a method that gets us quite close to the correct answer.

Rounding means that if the cents in a charge are less than 50 cents, we round them ?down? to 0; for example, a charge of $12.30 rounds down to $12. If the cents in the charge are equal to or greater than 50 cents, we round them ?up? to the next dollar; for example, a charge of $12.80 rounds up to $13. Now all the charges in our list to add up are in dollars?much easier to manage!

Here is an example of how to round your shopping list:

  • $25.17 ? Because 17 cents is less than 50, round the charge down to $25
  • $14.87 ? Because 87 cents is more than 50, round the charge up to $15.
  • $23.12 ? Rounds down to $23.
  • $16.72 ? Rounds up to $17.

Check it! \begin{align*}\$25 + \$15 + \$23 + \$17 = \$80\end{align*}.

Now try adding the exact charges. \begin{align*}\$25.17 + \$14.87 + \$23.12 + \$16.72 = \$79.88\end{align*}.

Using rounding, the total is only off by 12 cents, and the calculation is much faster to do in your head.

As the cashier is adding up your charges, you can use rounding to check whether she entered the correct numbers. If the total you obtain is more than $1-2 off the total she calculated, it might be worth asking her to double check!

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The prices of Charlene?s purchases are as follows:

  • $12.24
  • $  7.30
  • $  4.89
  • $22.50
  • $12.34

Round each of the charges and find the total cost of Charlene?s purchases without a calculator.

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