Lesson 4 β€” Activity 2: Adding and Subtracting Decimal Numbers

Getting Ready

Think back to the previous lessons where you practised adding and subtracting whole numbers and to the activity on rounding decimal numbers that you just completed. These skills will be used as you learn about adding and subtracting decimal numbers.

As you complete the calculations for decimal numbers in this activity, you will check your answers for reasonableness using rounding. What is reasonableness?

Try This:

Look at the headline below. Think about how realistic or reasonable it is. Is there any reason you think it may not be true?

              Image 1

Girl Finds 1,000,000 Dollar Coins in a Suitcase

If you estimate the amount of space needed to hold 1,000,000 dollar coins, you can figure out that it’s not very reasonable that all that money would fit in a suitcase.

There are three ways to add and subtract decimals:

  • paper and pencil method 

  • using a calculator (and for verifying answers)

  • using drawings or manipulatives

1. Paper and Pencil Method

  • Line up the decimals when adding and subtracting to put the digits in their correct place value column (e.g., tens above tens, tenths above tenths).


         Addition:               Subtraction:
         $12.34                       $ 38.84
         +18.51                        – 24.51
        $30.85                        $14.33

Round to the nearest whole number to check for reasonableness:

For the addition question: $12.34 rounds to $12 and $18.51 rounds to $19, therefore

$12 + $19 = $31 (You can see your calculation is reasonable.)

For the subtraction question: $38.84 rounds to $39 and $24.51 rounds to $25, therefore

$39 - $25 = $14 (You can see again that your calculation was reasonable.)

2. Use a Calculator

  • Use the decimal key on your calculator to place a decimal in a number.
  • Here is the addition question:

This image shows how to calculate decimal numbers with a calculator.

3. Use Drawings or Manipulatives

Here's an example:
Sarah has 3 dimes, 9 nickels, and 2 pennies. You can use base 10 blocks, manipulatives, or a drawing to figure out how much money she has.

Images courtesy of K&E Studio