Estimating Decimal Products

Estimating Decimal Products

Students will be able to fluently multiply multi-digit decimals using the standard algorithm for each operation.

How can you use estimation to find a product?

The students at Chapel Glen are selling tins of popcorn for $9.25 per tin to raise money for new recess equipment. They sold 42 tins in the first week. Estimate how much money the students have raised selling popcorn in the first week.

There are two methods we can use...

rounding each factor

OR

using compatible numbers

ONE WAY - Round Each Factor

• Round 42 down to 40

• Round $9.25 down to $9

• Multiply 42 x 9 ≈ $360

• The students raised approximately $360 the first week!

ANOTHER WAY - Compatible Numbers

• Keep 42 the same

• Round $9.25 to $10 because it's easy to multiply by 10

• 42 x $9.25 ≈ $420

• The students raised about $420 the first week!

Let's try one together!

We will estimate 7.83 x 3.8 by rounding each factor

Multiple Choice

What should we round 7.83 to?

• 7

• 7.5

• 8

• 10

Multiple Choice

What should we round 3.8 to?

• 3

• 3.5

• 4

• 38

Let's estimate!

8.0 x 4.0 ≈ ?

Multiple Choice

What is 8 x 4?

• 24

• 26

• 30

• 32

So 7.83 x 3.8 ≈ 32

Let's try one more!

We will estimate 44.3 x 6.71 by using compatible numbers

Multiple Choice

Which number would be best to make a compatible number?

• 44.3

• 6.71

Why?

We typically feel most comfortable multiplying with numbes less than twelve. Since 44.3 is a larger number, we may want to use a compatible number in its place.

Multiple Choice

What compatible number should we use for 44.3?

• 30

• 40

• 50

• 100

Multiple Choice

We can also round 6.71 to make it easier. What number should we round 6.71 to?

• 6

• 6.5

• 7

• 7.5

Let's estimate!

40 x 7 ≈ ?

Multiple Choice

40 x 7 = ?

• 280

• 290

• 300

• 310

So 44.3 x 6.71 ≈ 32

Now you try!

We'll still break it down, but we will work through each one by using the ROUNDING method.

Multiple Choice

Round: 6.8 x 53

• 7 x 53

• 6 x 50

• 7 x 50

• 68 x 53

Multiple Choice

7 x 50 = ?

• 150

• 250

• 350

• 450

Multiple Choice

Round: 2,386.25 x 40.1

• 2,400 x 40

• 2,300 x 40.1

• 2,300 x 41

• 2,000 x 40

Multiple Choice

2,400 x 40 = ?

• 9,600

• 94,940

• 95,000

• 96,000

Now you try it ALL!

Round then solve to find out your approximate answer!

Multiple Choice

65.13 x 2.89 ≈ ?

• 70

• 179

• 195

• 203

Multiple Choice

518 x 6.82 ≈ ?

• 3,500

• 3,550

• 3,600

• 4,000

Now let's try compatible numbers!

We'll still break it down, but we will work through each one by using the COMPATIBLE NUMBERS method.

Multiple Choice

Compatible numbers for 9.34 x 0.68

• 9 x 1

• 9 x 0

• 9 x 0.5

• 9 x 68

Multiple Choice

9 x 1 = ?

• 5

• 6

• 7

• 9

Multiple Choice

Compatible numbers for 52.3 x 9.7

• 52 x 9

• 50 x 10

• 52.3 x 50

• 523 x 97

Multiple Choice

50 x 10 = ?

• 50

• 500

• 5,000

• 5,010

Now you try it ALL!

Use compatible numbers to find out your approximate answer!

Multiple Choice

35.7 x 8.9 ≈ ?

• 360

• 365

• 370

• 375

Multiple Choice

20.6 x 3.7 ≈ ?

• 20

• 40

• 80

• 160

That's it!

Share how you feel about estimating the products of decimals on the next slides.

Poll

Which method did you like more?

• ROUNDING EACH FACTOR

• USING COMPATIBLE NUMBERS

Poll

How do you feel about estimating the products of decimals?

• GREAT

• OKAY

• NOT GREAT

Poll

Would you like to be in a small group to improve your skills on estimating the products of decimals?

• YES

• NO