ESTIMATING DECIMALS SUMS AND DIFFERENCES

Visual Model

Problem 1 :

4.36 + 2.87

Solution:

4.36 + 2.87 ≈ 4.5 + 3.0

          = 7.5

Problem 2 :

9.78 - 5.46

Solution:

9.78 - 5.46 ≈ 10.0 - 5.5

        = 4.5

Problem 3 :

5.13 - 3.58

Solution:

5.13 - 3.58 ≈ 5.0 - 3.5

          = 1.5

Problem 4 : 

2.94 + 4.08 

Solution:

2.94 + 4.08 ≈ 3.0 + 4. 0

         = 7.0

Problem 5 :

Approximate the total length of cheetah.

Solution:

3.21 + 2.83 ≈ 3.0 + 3.0

= 6.0

The cheetah's total length is about 6 feet. 

Problem 6 :

3.65 - 2.72 ≈ _____

Solution:

3.65 - 2.72 ≈ 3.5 - 2.5

= 1

Problem 7 :

9.03 - 6.78 ≈ _____

Solution:

9.03 - 6.78 ≈ 9.0 - 7.0

= 2

Problem 8 :

2.35 + 5.67 ≈ _____

Solution:

2.35 + 5.67 ≈ 2.5 + 5.5

= 8

Problem 9 :

8.21 - 4.11 ≈ _____

Solution:

8.21 - 4.11 ≈ 8.0 - 4.0

= 4

Problem 10 :

5.68 + 4.38 ≈ _____

Solution:

5.68 + 4.38 ≈ 5.5 + 4.5

= 10

Problem 11 :

3.92 + 3.92 ≈ _____

Solution:

3.92 + 3.92 ≈ 4.0 + 4.0

= 8

Problem 12 :

2.61 + 3.45 ≈ _____

Solution:

2.61 + 3.45 ≈ 2.5 + 3.5

= 6

Problem 13 :

2.07 - 1.45 ≈ _____

Solution:

2.07 - 1.45 ≈ 2.0 - 1.5

= 0.5

Problem 14 :

10.04 - 6.79 ≈ _____

Solution:

10.04 - 6.79 ≈ 10.0 - 7.0

= 3

Approximate the total cost of the two shirts.

Problem 15 :

Solution:

The total cost of two shirts = $17.89 + $15.07

≈ $18.0 + $15.0

= $33

Problem 16 :

Solution:

The total cost of two shirts = $23.49 + $23.49

≈ $23.5 + $23.5

= $47

Problem 17 :

At the grocery store, you buy items for $1.79, $3.15, $2.45, $9.08 and $3.49. Estimate the total amount you spend. _____

Solution:

$1.79 ≈ $2.0

$3.15 ≈ $3.0

$2.45 ≈ $2.5

$9.08 ≈ $9.0

$3.49 ≈ $3.5

Total amount = 2.0 + 3.0 + 2.5 + 9.0 + 3.5

= $20

Problem 18 :

You buy 6 hamburgers that cost $3.45 each. Using only mental math, estimate the total amount of the 6 hamburgers.

Solution:

$3.45 ≈ $3.5

One hamburger = $3.5

6 hamburgers = 6 × $3.5

= $21