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### Unit 3: Lesson 3

Adding and subtracting fractions with like denominators

# Adding fractions with like denominators

Sal adds 3/15+7/15. Created by Sal Khan and Monterey Institute for Technology and Education.

## Want to join the conversation?

• Does GCD mean greatest common divider. If so than how is it any different from LCM?
• Greatest common denominator
• Why do the denominators have to be the same?
• The denominators have to be the same so you can add the numerators together without worrying about the denominators being different sizes, because that affects the value of the fraction.

It's like trying to count the number of pieces you can get out of different cakes. If the cakes are different sizes, it wouldn't be fair because some people would get larger pieces than others. By making cakes that are the same size and then counting the number of pieces, you can be sure that everyone is getting a fair amount of cake.

In order to add fractions correctly, the "cakes" need to be the same size, which is why the denominators (bottom numbers in the fractions you are adding) need to be the same value.
• at , Sal said that if the denominator is the same, you just add the numerator and then the denominator stays the same? I just dont get it. Is it the same on the problems without the same denominators?
• you just add the top and the bottom stays the same.
• What if your sum goes over the denominator? Say, 7/14 + 9/14. How do you deal with that?
• you still add the numerator together. Like 16/14.But this will become an improper fraction, the actual answer is 1 2/14. A mixed fraction.
• Does GCF mean greatest common factor ?
• Yes that is what it stands for
• Does the GCM equal the LCM when the BCM Is divided by the WCM?
• GCM is a greatest common multiplier and LCM is a least common multiplier. They are different.
• What would you do if a mixed number had an improper fraction in it?
• it is not completely simplified if a mixed number has an improper fraction in it