This document provides instructions for adding and subtracting decimals. It outlines three main rules: 1) line up decimals by adding zeros as placeholders, 2) bring the decimal straight down, and 3) only add or subtract like columns. It demonstrates adding and subtracting decimals through examples like 7.1 + 5.8 + 2.5 and 34 - 1.2. The document also introduces a front-end estimation technique for quickly estimating decimal sums and differences by rounding decimals and adding front-end digits. An example shows using this technique to estimate if three video game prices can be purchased using a $25 gift certificate.

1. ADDING AND
SUBTRACTING
DECIMALS
SECTION 1.07

2. RULES
• LINE UP DECIMALS
– If you can not drop a straight line down
through your decimals, they are not lined up
correctly
– Add zeros as placeholders if needed
• BRING YOUR DECIMAL STRAIGHT DOWN
• ADD OR SUBTRACT AS USUAL

3. Add & Subtract like Columns
7.1 + 5.8 + 2.5 = _______
ones
RULE: Only add
and subtract
LIKE columns.
1
7.1
5.8
+ 2.5
15 . 4
tenths

4. Make The Columns Even
34.5 + 21 + 8.5
RULE: Add zeros to
even the columns
34.5
21.0
+ 8.5
34.5
21.0
+ 8.5

5. Make The Columns Even
44.589 + 7.23 + 5
3 decimal 2 decimal
columns columns
0 decimal
columns
1
1
44.589
7.230
+ 5 .000
56.819

6. Subtract Similar Columns
34 – 1.2 = _____
ones
tenths
34. 0
- 1. 2
32 . 8
33

8. FRONT-END ESTIMATION
• Adding the numbers in front of the
decimals
• Rounding the decimal numbers to the
nearest whole number
• Add the two sums

9. EXAMPLE
•
For your birthday you receive a $25 gift certificate. You
want to buy 2 used video games, whose prices are:
$8.79, $7.29, $ 7.89 Can you buy all 3 games using the
gift certificate?
1. Add the front-end digits
$8 + 7 + 7 = $22
2. Estimate the sum of the remaining digits (what is
behind the decimal)
.79 and .29 are about $1
.89 is about $1
$1 + $1 = $2
3. Add the results.
$22 + $2 = $24
The estimated sum is less then $25, so you can buy all 3
with your gift card.