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For more information, please see full course syllabus of Pre Algebra
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Lecture Comments (10)

0 answers

Post by Terrance Goins on July 24, 2015

why when subtracting fractions you borrow at times and sometimes you treat them as integers how do you know when to use the proper operation?

0 answers

Post by Mohammed Jaweed on July 22, 2015

3 times 9 doesn't equal 18 it equals 27.

0 answers

Post by Habida Popal on July 17, 2014

5/9-3/5=-2/45...not -1/8....please check your answers and work carefully. Otherwise, I really like following your examples.

0 answers

Post by Daniel Eckert on April 29, 2014

5/9 - 3/5 = 25/45 - 27/45= -2/45 right? I notice 95% of the questions don't get replies or it takes 2 years to get a response but I would like to know the right answer.

1 answer

Last reply by: Professor Fung
Mon Jul 8, 2013 1:21 AM

Post by Omar Arab on July 8, 2013

You are explaining a very fast.

0 answers

Post by denny beltre on October 8, 2012

you get the idea which is what's important.

0 answers

Post by RAFAEL MENDIVES on June 19, 2011

5/9 - 3/5 = 2/45. the video show wrong answer

1 answer

Last reply by: Jesse Ogwal
Fri Aug 31, 2012 9:21 AM

Post by earl west on February 1, 2011

in example 3/4-7/8 ; note that 3x9=27 not 18.

Subtracting Rational Numbers

  • To subtract fractions with different denominators, write equivalent fractions for each fraction so that have the same denominator. Then subtract the numerators and keep the denominators the same. Remember to always check if you can simplify the fraction as your last step.
  • Any common denominator can be used. If you are having trouble finding a common denominator you can multiply the denominators of the fractions with each other. However, using the Lowest Common Denominator may save steps and make simplifying the difference a lot easier.
  • To subtract rational numbers that are in different forms, either make all the numbers fractions or make all the numbers decimals.

Subtracting Rational Numbers

[7/9] − [3/5] =
  • [35/45] − [27/45] =
[8/45]
[6/7] − [1/3] =
  • [18/21] − [7/21] =
[11/21]
− [8/15] − [1/6] =
  • − [16/30] − [5/30] =
  • − [21/30] =
− [7/10]
[34/8] − 2.2 =
  • 4[2/8] − 2[2/10] =
  • 4[10/40] − 2[8/40] =
  • 2[2/40] =
2[1/20]
9.55 − 5[1/4] =
  • 9[55/100] − 5[1/4] =
  • 9[55/100] − 5[25/100] =
  • 4[30/100] =
4[3/10]
[67/25] − 2.1 =
  • 2[17/25] − 2[1/10] =
  • 2[34/50] − 2[5/50] =
2[29/50]
5.09 − 2[3/8] =
  • 5[9/100] − 2[3/8] =
  • 5[18/200] − 2[75/200] =
  • 4[218/200] − 2[75/200] =
2[143/200]
One watermelon weighs 10.4 pounds. Another watermelon weighs 10[7/9] pounds. How much more does the heavier watermelon weigh?
  • 10.4 = 10[4/10] = 10[36/90]
  • 10[7/9] = 10[70/90]
  • 10[70/90] − 10[36/90] =
[34/90] pounds
A plant was 5[1/12] inches tall last year. Now, the plant is 17.29 inches tall. How much did the plant grow in the past year?
  • Let g = inches the plant grew in the last year
  • 5[1/12] + g = 17.29
  • g = 17.29 − 5[1/12]
  • 17[29/100] − 5[1/12] =
  • 17[348/1200] − 5[100/1200] =
  • 12[248/1200] =
12[31/150] inches

*These practice questions are only helpful when you work on them offline on a piece of paper and then use the solution steps function to check your answer.

Answer

Subtracting Rational Numbers

Lecture Slides are screen-captured images of important points in the lecture. Students can download and print out these lecture slide images to do practice problems as well as take notes while watching the lecture.

  • Intro 0:00
  • What You'll Learn and Why 0:06
    • Topics Overview
  • Vocabulary 0:19
    • Least Common Denominator (LCD)
  • Subtracting with Unlike Denominators 0:41
    • Example: 5/9 - 3/5
    • Example: 3/4 - 7/8
  • Subtracting Rational Numbers 1:59
    • Example: 23/4 - 3.5
    • Example: 11.7 - 3/4
  • Subtracting Rational Numbers in Word Problems 4:37
    • Puppy's Weight
  • Extra Example 1: Subtracting with Unlike Denominators 6:48
  • Extra Example 2: Subtracting Rational Numbers 7:27
  • Extra Example 3: Rainfall 10:32
  • Extra Example 4: Decorating Your House 12:06