Mental Math
Estimating/Rounding
Using Models
Adding & Subtracting Decimals
Grab Bag
100
Use mental math to find the sum of $0.96 & $1.54
Explain which strategy you used
$2.50
Compensation:
$0.96 + $0.04 = $1.00
$1.54 - $0.04 = $1.50
100
Estimate the answer.
558 + 623
558 is about 600
623 is about 600
600 + 600 = 1,200
100
Give Ms. Entingh one example of a model you can use to help you add or subtract decimals.
Hundredths grid, Thousandths grid, base 10 blocks, bar diagram, etc.
100
Why is it helpful to estimate an addition or subtraction problem before finding the actual answer?
Because an estimate helps you decide if your answer is reasonable or not depending on how close your answer is to the estimate.
100
Dan built a building that was 145.89 feet high & Lisa built a building that was 128.06 feet high.
What is the combined height of their buildings?
145.89 + 128.06 = 273.95 feet high
200
Use the Commutative Property to solve the following problem: 11.45 yards + 3.39 yards = ?
Explain how the Commutative Property works for this problem.
Commutative Property: You can add decimals in any order.
11.45 + 3.39 = 3.39 + 11.45
So, 11.45+3.39 = 14.84 yards
200
If a question about rounding decimals does not tell which place to round to, what do you round the decimal to?
You round the decimal to the nearest whole number.
200
Model the following problem on a hundredths grid:
0.1 + 0.73 = 0.83
10 blocks in the hundredths grid should be shaded one color
73 blocks in the hundredths grid should be shaded a different color/pattern
200
Estimate, then solve:
11.39 pounds + 14.27 pounds
Is your answer reasonable, why or why not?
Estimate: 11 + 14 = 25 pounds
Solve: 11.39 + 14.27 = 25.66 pounds
Explain: Yes, my answer is reasonable because my answer is less than 1 away from the estimate
200
A rectangular classroom is 30.2 feet long and 25.89 feet wide.
What is the perimeter of the classroom?
30.2 + 30.2 + 25.89 + 25.86 = 112.18 feet
300
What does the Associative Property allow you to do
A. Change the grouping of addends
B. Add decimals in any order
C. Adjust one or both numbers to make the calculation easier.
A. Change the grouping of addends
300
Estimate the difference.
418.5 - 23.7 = ?
418.5 is about 419
23.7 is about 24
419-24=395 (Acceptable answers: 390 to 400)
300
Model the following problem using a hundredths grid:
1.33 - 0.35 = 0.98
133 blocks should be shaded on the hundredths grids
35 of the shaded blocks should be crossed out
300
Estimate, then solve:
16.82 inches - 5.21 inches
Is your answer reasonable? Why or why not?
Estimate: 17 - 5 = 12 inches
Solve: 16.82 - 5.21 = 11.61 inches
Explain: Yes, my answer is reasonable because my answer is less than 1 away from the estimate.
300
A rectangular dining room is 10.891 feet long and 12.56 feet wide.
Round the length and width to the nearest tenth. Then estimate the perimeter of the dining room.
10.891 = 10.9 ft
12.56 = 12.6 ft
10.9 + 10.9 + 12.6 + 12.6 = 47 feet
400
What do we call it when we adjust one or both numbers to make the calculation easier?
Compensation
400
Find the answer by rounding to the nearest hundredth.
52.096 - 12.555 = ?
52.10 - 12.56 = 39.54
400
Write an expression that is represented by the model:
0.20 + 0.20 + 0.20 = 0.60
0.2 + 0.2 + 0.2 + = 0.6
400
Tanya bought 2.3 pounds of peaches. She used 0.54 pound in a pie. How much of her peaches are left?
2.3 - 0.54 = 1.76 pounds are left
400
Choose all the expressions that are equal to 4.07 + 3.48
2.65 + 5.89
10 - 3.62
2.65 + 4.9
12 - 4.45
4.07 + 3.48 = 7.55
2.65 + 5.89 = 8.54
10 - 3.62 = 6.38
2.65 + 4.9 = 7.55
12 - 4.45 = 7.55
500
What are Compatible Numbers?
Numbers that are easy to compute mentally.
(Usually end numbers that add up to "10")
Example: 11.45+9.55+21
11.45 & 9.55 are compatible because the 5's in their hundredths places = 1 tenth
500
Gloria road her bicycle 2.3 miles on Sunday, 1.6 miles on Monday, and 3.5 miles every other day that week. About how many miles did she ride in all?
Gloria rode her bike about 24 miles in all.
2 + 2 + 4 + 4 + 4 + 4 + 4 = 24
500
Write an expression that is represented by the model:
0.86 - 0.23 = 0.63
500
Solve the following problem, then draw a bar diagram to represent the problem.
Thomas would like to take a drawing class. The class costs $250. He has saved $62.80 so far. How much more money does Thomas need to save?
250 - 62.80 = $187.20
500
Choose Yes or No to tell if the number 3.09 will make each equation true.
1) 6.21 + ? = 9.30
2) ? + 4.87 = 8.07
3) 9.21 - ? = 5
4) ? - 0.75 = 2.34
1) Yes
2) No
3) No
4) Yes