6th Grade Math Textbook, Fundamentals

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4-4

Key Concept

Rounding Rules

• Find the place to which you are rounding.


• Look at the digit to the right of that place. If the digit is less
  than 5, the digit being rounded stays the same. If the digit
  is 5 or greater, the digit being rounded increases by 1.

So the seventh grade class collected about 49.5 kg or about 50 kg

of aluminum for recycling.

Estimates get closer to the exact answer as the place value of the digit

being rounded decreases. So rounding to the nearest tenth gives an

estimate closer to the exact answer than rounding to the ones place.

Either estimate could be helpful, however, depending on its purpose.

Estimate the difference: 589.483 24.678

Method 1 Round each addend to the

nearest whole number.

14.836 8 5, so 4 rounds to 5 ones.

8.541 5 5, so 8 rounds to 9 ones.

26.1 78 1 5, so 6 stays as 6 ones.

14.836 15

8.541 9

26.178  26

50

Method 1 Round to the Nearest Ten

589.483 9 5, so 8 rounds to 9 tens.

24.678 4 5, so 2 stays as 2 tens.

589.483 590

24.678  20

570

So, when each addend is rounded to the nearest ten, 589.483 24.678 570.

Method 2Round each addend to a specific

decimal place value—for example,

to the nearest tenth.

14. 836 3 5, so 8 stays as 8 tenths.
    8.541 4 5, so 5 stays as 5 tenths.
    26.1 78 7 5, so 1 rounds to 2 tenths.

14.836 14.8

8.541 8.5

26.1 78 26.2

49.5

Remember:

The symbol means

is approximately equal to.

Update your skills. See pages 408 III, 409 V.

Estimate Decimal Sums

and Differences

Objective To estimate decimal sums and differences

using rounding, front-end, or clustering techniques

The seventh grade class at Park School collects aluminum for
recycling. The students collected 14.836 kg of aluminum the first
month, 8.541 kg the second month, and 26.178 kg the third month.
About how many kg of aluminum did the seventh grade class
collect over the three-month period?

Estimation can provide a quick answer

when an exact answer is not necessary.

It is also a way to check whether or

not an exact answer is reasonable.