Let’s estimate when we Let’s estimate when we multiply decimal fractions!multiply decimal fractions!

This is one way to check our This is one way to check our products.products.

Find 12 x 0.48 by rounding.Find 12 x 0.48 by rounding.

Round the 12 to 10Round the 12 to 10 Round the 0.48 to .5Round the 0.48 to .5 Now, let’s multiply.Now, let’s multiply.

10 x .5 = 5.010 x .5 = 5.0

Our answer should be near 5 Our answer should be near 5 according to our estimation.according to our estimation. Let’s multiply:Let’s multiply:

0.480.48

x 12x 12

9696

+ 480+ 480

5.765.76

Since the estimate is 5, place the decimal Since the estimate is 5, place the decimal point so there is a one-digit whole point so there is a one-digit whole number in the product. number in the product.

Count Decimal PlacesCount Decimal Places

Let’s multiply:Let’s multiply: 0.0.4848 x 12x 12

9696 + 480+ 480

5.5.7676Since we had two decimal places in Since we had two decimal places in

our factors, we have to have two our factors, we have to have two places in our product.places in our product.

Let’s count decimal places, Let’s count decimal places, again.again. Find the product of 0.7 x 0.5Find the product of 0.7 x 0.5 Multiply as you do with whole Multiply as you do with whole

numbers.numbers. 0.70.7 x 0.5x 0.5 0.350.35Count the decimal places in each Count the decimal places in each

factor. There are 2 decimal places in factor. There are 2 decimal places in the factors. We need 2 decimal the factors. We need 2 decimal places in the product.places in the product.

Your turn.Your turn.

Estimate by rounding.Estimate by rounding.

0.4 x 1.7 =0.4 x 1.7 =

What should you do first?What should you do first?

EstimationEstimation

Let’s round 0.4 to 0.5Let’s round 0.4 to 0.5 Next, round 1.7 to 2Next, round 1.7 to 2 Now, let’s multiply.Now, let’s multiply.

0.5 x 2 = 1.00.5 x 2 = 1.0

Let’s check our answer. It should Let’s check our answer. It should be near 1.be near 1.

Count the decimal places to Count the decimal places to multiply.multiply. Multiply:Multiply:

..44 x 1.x 1.77

2828 4040

..6868There are 2 decimal places in our factors. We There are 2 decimal places in our factors. We

have to have 2 decimal places in our have to have 2 decimal places in our product. product. .68 .68 would roundwould round toto 1.0 1.0

Your turn again.Your turn again.

How many decimal places will How many decimal places will be in the product for these be in the product for these factors?factors?

2.5 x .072.5 x .07 Count .5 and .07Count .5 and .07 There will be 3 decimal places in There will be 3 decimal places in

the product.the product.