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Lesson 5: Adding and subtracting fractions with unlike denominators word problems

Learn how to add and subtract fractions with unlike denominators through a real-world problem. Watch as the problem is broken down step-by-step, practice finding common denominators, and apply this knowledge to determine if the sum or difference of the fractions meets a specific requirement. Created by Sal Khan.

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• question when looking for a common denominator
must it be the lowest you can find or can it be just any?
(1 vote)
• A common denominator cannot be "any" number, because it has to fit both fractions. However, it can be "any" fitting number. The reason you want it to be a smaller number is that it makes adding and simplifying easier. (see simplifying fractions) I hope this helped!
• you are confusing me about splitting the bar graph
• Splitting the bar graph really isn't at all confusing.It means that you divide something into fractions so you can understand better.The best object to use is a bar graph.Sal divided the first two into different fractions.Then, when you converted the two into fractions with same denominators,you add them together.When you add it,you get the answer,which Sal demonstrated in the third bar graph. So it is not confusing at all.Thanks for reading this reply!
• thank u for helping me i think ima get a 100
• Hope you do
• if you are confused in this vid comment
• If you are watching all the videos and taking all the tests and quizzes you must be able to get comfy w these. Don't forget to work hard and be patient.
• "Since 5 and 2 are both prime numbers, the smallest number (least common multiple) is just going to be their product."

Is there an explanation for this?
• Feels good revising this again, I honestly did this in grade 6.
(1 vote)
• Yo...this is kinda hard. :? But I get it anyways.
(1 vote)
• How come when you have prime numbers, the smallest common multiple is them times each other? (Example: 5 and 2.) At - , Sal said that when both numbers were prime, you just needed to multiply them times each other. Why does that work?
(1 vote)
• That works because the 2 numbers he's using have only 2 factors each(1&5) and (1&2) and since these 2 numbers have only these factors or they are considered prime,you will find that the smallest common multiple is just multiplying those 2 numbers together.
(1 vote)
• thanks for the help. But when i am doing a harder problem is there an easy and quick way to check if you got it right?
(1 vote)
• You can estimate. Sometimes this doesn't work, but it usually does. Sal, at the end of the video, estimated to check his answer (2/5 is less than half, so when added 1/2, it won't equal a whole).
(1 vote)