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### Course: Arithmetic (all content)>Unit 5

Lesson 21: Multiplying fractions

# Intro to multiplying 2 fractions

Sal introduces multiplying 2 fractions.  Created by Sal Khan.

## Want to join the conversation?

• Is there another way to put answers in simplest form another way then the ones shown in the video?
• I personally think it is easier to simplify a fraction by dividing the numerator and the denominator by their lowest common factor, eg.

4/8

LCM = 4

4 divide 4 = 1
8 divide 4 = 2

= 1/2

Or...

6/8

LCM = 2

6 divide 2 = 3
8 divide 2 = 4

= 3/4

Hope that helps.
• 1/20 x 2 11/20=
• 0.1275 would be the answer
• Hello!
I don't really understand why we are splitting 2/3 into 4/5th. I thought splitting things is considered division?
Thank-you!
- Andy An
• Fractions can be confusing.That step is division, but the equation is not.When you multiply with whole numbers, you take one number and make it grow, but since fractions are less than one,they have strange properties that basically switch the function of multiplication and division.If you didn't understand this and/or this didn't help you, rewatch the video and pay close attention.
• So 5/8 x 4/10 would be 1/4
• Yes.
5/8 and 4/10 would be 5 * 4 or 20 in the numerator, and 8*10 or 80 in the denominator. 20/80 can be reduced to 2/8 and then 1/4.
• Aren't we supposed to multiply both the denominators till they have an equivalent value? Or is it supposed to be until they have a common denominator?
• We only need a common denominator to add/subtract fractions. We do not use a common denominator to mulitply/divide fractions.
• I understand how you do it and I can follow the instructions to get the desired result.

However, can someone explain why when we multiply the 2 fractions together we are 'taking 2/3 of 4/5'? I've never thought of multiplication in this way before.

2/3 x 4/5 = 8/15
but
2/3 & 4/5, when changed using the LCM, would be 10/15 & 12/15. Larger fractions than the 8/15. I always thought that multiplication would increase our final result, but this is not the case?

So in terms of multiplication with fractions, I should view it as taking a portion? Example - 2/3 OF 4/5?

Thanks to anyone that answers me :)
• Yes, we are taking a portion due to the fact that we are multiplying fractions, which are less than one.For more information, see my reply to Andy's post.
• how do you simifly farctions
• It has a few spelling errors. But other than that, good question!
• When we multiplying fractions, why do we multiply the numerators and the denominator?
• Do you always need to simplify?
• It's not necessary, but it is really helpful for when you need a reduced fraction.
• When you make the statement, "If you have 12 of something and want to take 2/3 of it, you are going to take 8," you have done two WRONG things. First, you stop looking at the problem from the visual viewpoint. Second, you throw out a mathematical statement with no evidence, no explanation of how you got to the fact of that statement. I am left totally clueless from either perspective. I cannot do these types of problems. Where do you explain any of it from the beginning and consistently?
• So, let's say that you have twelve of something, right?
Part 1:
If you want to take 2/3's of 12,
we should find 1/3 of 12 first so that we can multiply 1/3 of 12 and 2 which will get us 2/3's of 12,
because 2/3 is 1/3 times 2.
It works the same way for other fractions:
2/7 times 2 is 4/7
3/8 times two is 6/8
To find 1/3 of 12, simply multiply 12 by the numerator of 1/3(which is 1), and divide 12 by the denominator(which is 3).
Part 2:
Doing the operations I stated in the last sentence:
Multiply 12 by the numerator(which is 1): 1 times 12 = 12
Divide the number we got by the denominator(which is 3): 12 / 3 = 4
4 is 1/3 of 12. So, to find 2/3 of 12, we multiply 4 by 2
Part 3:
4 * 2 = 8
8 is 2/3 of 12!