Mental Math
Estimate Sums and Differences
Add and Subtract Decimals
Add Decimals
Subtract Decimals
Model with Math
100
Use mental math to solve: 5.7 + 4.3 + 6
sample answers:
0.3 + 0.7 = 1.0, then 1 + 5 + 4 + 6 = 16
or....5.7 + 4.3 = 10, 10 + 6 = 16
100
Estimate the sum:
3.8 + 4.1 + 3.3
3.8 rounds up to 4
4.1 rounds down to 4
3.3 rounds down to 3
4 + 4 + 3 = 11
100
Write an expression that is represented by the model below:
1.45 - 0.31
100
Find the sum
1.03 + 0.36
1.39
100
Find the difference.
13.9 - 3.8
10.1
100
Karina saved $12 in March, $15.75 in April, and $10.90 in May. Then she bought a shirt for $23.60. How much money does Karina have left?
$12 + 15.75 + 10.90 = 38.65
38.65 - 23.60 = 15.05
Karina has $15.05 left.
200
Use mental math to solve: 275 + 180 + 120
180 + 120 = 300, then 300 + 275 = 575
200
Estimate the difference:
788.9 - 572
788.9 rounds up to 800
572 rounds up to 600
800 - 600 = 200
200
Each set of place-value blocks below represents a decimal.
Write and solve the equation represented by the place-value blocks below.
0.32 + 0.75 = 1.07
200
Find the sum.
45.08 + 2.01
47.09
200
Find the difference.
8.04 - 0.3
7.74
200
Allure has $15. She buys a ticket to a movie for $9.50 and a smoothie for $2.85. How much money does she have left?
15.00 - 9.50 - 2.85 = $2.65.
She has $2.65 left.
300
Laila bought three books that cost $3.95, $4.99, and $6.05. How much did she spend in all?
She spent $ 14.99 in all.
300
Three rock samples have masses of 74.05 grams, 9.72 grams, and 45.49 grams. Anyobix estimates the total mass of the samples by rounding each mass to the nearest whole number.
What are the numbers when rounded to the nearest whole number?
74.05 = 74
9.72 = 10
45.49 = 45
300
Ariel collected rocks that has a mass of 54.14 grams. Kaydien started to collect rocks and so far has 8.98 grams. Betssy decided to join and collected 7.39 grams of rocks. What is the total grams of their collection? What is the difference between the heaviest mass and the lightest?
Total= 70.51 grams
Difference = 46.75 grams
300
Find the sum.
7.2 + 3.96 + 8.8
19.96
300
Find the difference.
34.49 - 12.61
21.88
300
For her birthday, Kyrah received $20 from her aunt, $15 from her grandmother, and $32 from her cousins. She bought an e-book for $10.85. How much birthday money does Kyrah have left?
20 + 15 + 32 = $67
67 - 10.85 = $56.15
Kyrah has $56.15 left.
400
Describe and give an example of the commutative property.
The Commutative Property lets you add two decimals in any order.
ex: $11.45 +$3.39 = $3.39 + $11.45
400
Gavin buys a game for $7.89 and some batteries for $5.49. He pays with a $20 bill. What is the best estimate of how much change he should get?
7.89 = 8; 5.49 = 5
5 + 8 = 13; 20 - 13 = 7
The best estimate is $7.00 in change.
400
Nevaeh gave Heaven a rug for her birthday. The rug is 14.72 ft wide and 24. 05 ft long. About how big is the perimeter of the rug to the nearest whole number?
+ 400 points: Is the estimate bigger or smaller than the actual perimeter? How much bigger or smaller?
about 78 ft long
The estimate is .46 ft bigger.
400
Traci earned $16 babysitting and $17.50 mowing a lawn. He paid $8.50 for a movie and bought a small popcorn for $1.95. Write an expression to show how much money he has left, and solve.
16 + 17.50 - 8.50 - 1.95 = $ 23.05
400
Ly'te bought a book for $35.60 and a set of pens for $12.78. How much did he spend in all? If he paid with three $20 bills , How much change did he get?
$48.38 = total amount he spent
$11.62 = his change from $60.00
400
Conrad paid $52.26 for a pair of shoes. Michelle paid $3.89 more for a pair of shoes than Conrad paid for his.
How much did Michelle pay for her shoes? Use the bar diagram to help you.
Michelle paid $56.15 for her shoes.
500
Describe and give an example of the Associative Property.
The Associative Property lets you change the grouping of addends.
ex: ($11.45 +$3.39)+$9.55 =$11.45 +($3.39 +$9.55)
500
The cost of one DVD is $16.98, and the cost of another DVD is $9.29. Ed estimated the cost of the two DVDs to be about $27. Is his estimate higher or lower than the actual cost? Explain.
His estimate is lower because he rounded 16.98 up to 17 and 9.29 up to 10. The actual cost is $26.27.
500
Alondra put 3 shelves in her room.The top is 2.55 feet long. The middle is 2.25 feet long, and the bottom shelf is 3.88 feet long. What is the combined length of the 3 bookshelves? How much longer is the bottom shelf than the top shelf?
8.68 feet long = 3 shelves
The bottom shelf is 1.33 feet longer.
500
Choose all expressions that are equal to 12.9.
500
Mathias bought 8.6 lbs. of strawberries, Joss bought 40.44 lbs of strawberries, Kevin bought 23.5 pounds of strawberries, and Prince bought 34.5 lbs. of strawberries. What is the difference between the amounts Mathias bought from the amount of each of the other people bought?
Joss bought 31.84 lbs. more strawberries than Mathias.
Kevin bought 14.9 lbs more strawberries than Mathias.
Prince bought 25.9 more strawberries than Mathias.
500
Jayce has $30.50. He wants to buy his dog a sweater that costs $15, a toy that costs $3.79, and a leash that costs $14.79.
How much more money does he need?
15 + 3.79 + 14.79 = 33.58
33.58 - 30.50 = $3.08
Jayce needs $3.08 more.