Mental Math
Estimate Sums and Differences
Use Models to 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
Eva 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 Eva have left?
$12 + 15.75 + 10.90 = 38.65
38.65 - 23.60 = 15.05
Eva 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
Allison 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
Joanne 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. A scientist 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
Without adding the decimals, tell if the sum of 0.46 + 0.25 is less than or greater than 1? Explain.
The sum would be less than 1, because 0.46 is less than half, and 0.25 is less than half, so added together they would still be less than 1.
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, Sophia 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 Lucy have left?
20 + 15 + 32 = $67
67 - 10.85 = $56.15
Lucy 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
Umberto 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
Estimate to decide if the sum of 314 + 175 is more or less than 600.
Less than 600.
(314 rounds down to 300; 175 rounds up to 200; 300 + 200 =500.)
400
Tim 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
Jonah bought a 1.5-liter bottle of seltzer. He used 0.8 liter of seltzer in some punch. Which is greater, the amount he used or the amount he has left? Explain how you decided.
The amount he used is greater.
Sample answer: 1.5 - 0.8 = 0.7 and 0.8 > 0.7
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 higher because he rounded 16.98 up to 17 and 9.29 up to 10. The actual cost is $26.27.
500
Do you think the difference of 1.4 − 0.95 is less than or greater than 1? Explain.
Less than 1; 0.95 is almost 1, so the difference will be about 0.4.
500
Choose all expressions that are equal to 12.9.
500
Choose all of the subtraction problems with a difference of 1.65.
27.30 − 16.65
11.23 − 9.58
40.4 − 23.9
12.68 − 2.03
21.74 − 20.09
11.23 − 9.58
21.74 − 20.09
500
Tucker 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
Tucker needs $3.08 more.