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8.1 Decimals: rounding decimals

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This module is from Fundamentals of Mathematics by Denny Burzynski and Wade Ellis, Jr. This module discusses how to round decimals. By the end of the module students should be able to round a decimal number to a specified position.

Section overview

  • Rounding Decimal Numbers

Rounding decimal numbers

We first considered the concept of rounding numbers in [link] where our concern with rounding was related to whole numbers only. With a few minor changes, we can apply the same rules of rounding to decimals.

To round a decimal to a particular position:

  1. Mark the position of the round-off digit (with an arrow or check).
  2. Note whether the digit to the immediate right of the marked digit is
    1. less than 5 . If so, leave the round-off digit unchanged.
    2. 5 or greater . If so, add 1 to the round-off digit.
  3. If the round-off digit is
    1. to the right of the decimal point, eliminate all the digits to its right.
    2. to the left of the decimal point, replace all the digits between it and the decimal point with zeros and eliminate the decimal point and all the decimal digits.

Sample set a

Round each decimal to the specified position. (The numbers in parentheses indicate which step is being used.)

Round 32.116 to the nearest hundredth.

  • 1 is in the hundredths position of 32.116

  • The digit immediately to the right is 6, and 6 > 5 size 12{"6 ">" 5"} {} , so we add 1 to the round-off digit:

    1 + 1 = 2 size 12{"1 "+" 1 "=" 2"} {}

  • The round-off digit is to the right of the decimal point, so we eliminate all digits to its right.

    32.12

The number 32.116 rounded to the nearest hundredth is 32.12.

Round 633.14216 to the nearest hundred.

  • 6 is in the hundreds position of 633.14216

  • The digit immediately to the right is 3, and 3 < 5 size 12{"3 "<" 5"} {} so we leave the round-off digit unchanged.
  • The round-off digit is to the left of 0, so we replace all the digits between it and the decimal point with zeros and eliminate the decimal point and all the decimal digits.

    600

The number 633.14216 rounded to the nearest hundred is 600.

1,729.63 rounded to the nearest ten is 1,730.

1.0144 rounded to the nearest tenth is 1.0.

60.98 rounded to the nearest one is 61.

Sometimes we hear a phrase such as "round to three decimal places." This phrase means that the round-off digit is the third decimal digit (the digit in the thousandths position).

67.129 rounded to the second decimal place is 67.13.

67.129558 rounded to 3 decimal places is 67.130.

Practice set a

Round each decimal to the specified position.

4.816 to the nearest hundredth.

4.82

0.35928 to the nearest ten thousandths.

0.3593

82.1 to the nearest one.

82

753.98 to the nearest hundred.

800

Round 43.99446 to three decimal places.

43.994

Round 105.019997 to four decimal places.

105.0200

Round 99.9999 to two decimal places.

100.00

Exercises

For the first 10 problems, complete the chart by rounding each decimal to the indicated positions.

20.01071

Tenth Hundredth Thousandth Ten Thousandth
Tenth Hundredth Thousandth Ten Thousandth
20.0 20.01 20.011 20.0107

3.52612

Tenth Hundredth Thousandth Ten Thousandth
3.53

531.21878

Tenth Hundredth Thousandth Ten Thousandth
Tenth Hundredth Thousandth Ten Thousandth
531.2 531.22 531.219 531.2188

36.109053

Tenth Hundredth Thousandth Ten Thousandth
36.1

1.999994

Tenth Hundredth Thousandth Ten Thousandth
Tenth Hundredth Thousandth Ten Thousandth
2.0 2.00 2.000 2.0000

7.4141998

Tenth Hundredth Thousandth Ten Thousandth
7.414

0.000007

Tenth Hundredth Thousandth Ten Thousandth

Tenth Hundredth Thousandth Ten Thousandth
0.0 0.00 0.000 0.0000

0.00008

Tenth Hundredth Thousandth Ten Thousandth
0.0001

9.19191919

Tenth Hundredth Thousandth Ten Thousandth

Tenth Hundredth Thousandth Ten Thousandth
9.2 9.19 9.192 9.1919

0.0876543

Tenth Hundredth Thousandth Ten Thousandth

Calculator problems

For the following 5 problems, round 18.4168095 to the indi­cated place.

3 decimal places.

18.417

1 decimal place.

5 decimal places.

18.41681

6 decimal places.

2 decimal places.

18.42

Calculator problems

For the following problems, perform each division using a calculator.

4 ÷ 3 size 12{"4 " div " 3"} {} and round to 2 decimal places.

1 ÷ 8 size 12{"1 " div " 8"} {} and round to 1 decimal place.

0.1

1 ÷ 27 size 12{"1 " div " 27"} {} and round to 6 decimal places.

51 ÷ 61 size 12{"51" div " 61"} {} and round to 5 decimal places.

0.83607

3 ÷ 16 size 12{"3 " div " 16"} {} and round to 3 decimal places.

16 ÷ 3 size 12{"16 " div " 3"} {} and round to 3 decimal places.

5.333

26 ÷ 7 size 12{"26 " div " 7"} {} and round to 5 decimal places.

Exercises for review

( [link] ) What is the value of 2 in the number 421,916,017?

Ten million

( [link] ) Perform the division: 378 ÷ 29 size 12{"378 " div " 29"} {} .

( [link] ) Find the value of 4 4 size 12{4 rSup { size 8{4} } } {} .

256

( [link] ) Convert 11 3 size 12{ { {"11"} over {3} } } {} to a mixed number.

( [link] ) Convert 3.16 to a mixed number fraction.

3 4 25 size 12{3 { {4} over {"25"} } } {}

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Source:  OpenStax, Contemporary math applications. OpenStax CNX. Dec 15, 2014 Download for free at http://legacy.cnx.org/content/col11559/1.6
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