Download Decimal Operations – NOTES

TOPIC 2: ADDING & SUBTRACTING DECIMALS
MENTAL MATH
Compatible Numbers: Numbers that add up to values that are easy to
compute with mentally, like 10 or 100.
Example: 4 & 6, 7 & 3, and 8 &2 are pairs of compatible numbers because
they add up to 10.
Compensation: Adjust one number to make the calculation easier and then
make up for that change after the calculation.
Example:
36 + 27
Add 3 to the 27 to make it 36 + 30, which is 66. Then take the 3 away
again to get the actual sum…63.
ROUNDING
1. Determine the rounding place.
2. Figure out what two numbers your number falls between on a number
line.
3. Which of those numbers is your number closer to? (Is it more than
halfway or less than halfway to the next number?)
Example: Round to the nearest tenth………..26.3527
1. The rounding place is the tenths place….the place that the 3 is in.
2. My number is between 26.3 and 26.4 (the next tenth up) on a number
line.
26.3
26.3527
26.4
3. My number is more than halfway to the next number, so it would round
UP to 26.4.
Underline the digit
Look next door
If it’s 5 or above…add one more
If it’s less than 5…leave it for sure
Everything after is zeros to the core!
ESTIMATION:
1. Round the given numbers to values that are easy to compute with.
2. Then add or subtract the rounded numbers!
Example: Jackie earned $574.35 one week and $623.29 the following week.
About how much did Jackie earn altogether?
$574.35
$600.00
+ $623.29
+ $600.00
about $1200.00
MODELING
 Base 10 blocks can help us model addition and subtraction!!!
Example: 2.45 + 1.9
The sum is 3 flats, 13 rods, and 5 cubes. 10 of the rods can be traded
for another flat, giving us 4 flats, 3 rods, 5 cubes OR 4.35.
“THE OLD FASHIONED WAY” – The Traditional Algorithm
 Line up the decimal points!!! When adding or subtracting, you may only
add/subtract like things. So you can only add/subtract ones with ones, tenths
with tenths, hundredths with hundredths, etc. Lining up the decimal points
automatically lines up the place values correctly, so addition/subtraction
may be carried out just like with whole numbers.
 Fill in blank spaces with zeroes!!! Filling in blank spaces with zeroes helps
you keep everything lined up correctly and helps minimize calculation errors
(especially when borrowing is required in subtraction).