Staging content lifeboat
Course: Staging content lifeboat > Unit 14Lesson 14: Adding and subtracting fractions with like denominators
Subtracting fractions with like denominators
Sal subtracts 8/18-5/18. Created by Sal Khan and Monterey Institute for Technology and Education.
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- I dont like COVID-19 ;((48 votes)
- who doesn't honestly(11 votes)
- 4/5 divided by 2/5 equals 30/12 how is that wrong?(13 votes)
- 4/5 divided by 2/5 equals 4/5*5/2(because of reciprocal) which is 4/2 or 2. That's why it's wrong. Hope this helps!(33 votes)
- I have a hard time remembering and understanding what he says because he’s going way too fast…. Can there be a slower version please?(4 votes)
- Hi. Its Yagnesh here. if you don't understand what sal says, You need to do the following in order to slow down the video. you click on the setting button (looks like a wheel). then click on 'playback speed.'it will be normal. if you click on 0.75 or 0.5, it will become slower. Hope that helps.(14 votes)
- When subtracting fractions with the same denominators do we have to change the denominators?(5 votes)
- no the denominater stays the same(7 votes)
- 4/5 divided by 2/5 equals 30/12 how is that wrong(4 votes)
- why doesn't the denominators changes(4 votes)
- Becuase its subtracting with like denominators so they will not change they will stay the same because like means the same(3 votes)
- How is it possible to have a negative fraction? If you think about fractions as pieces of pie, wouldn't it be impossible to have negative pieces of pie?(4 votes)
- I understand your reasoning about that, but it doesn't quite work, with that logic you cant have negative whole number either, because you cant have a negative whole pie.
think of it like this; a fraction is a part of a number, right? well, you can have a negative part of a number as well as a negative number. look at this number line-
something has to go in between those spaces!
hope this helps!(4 votes)
- What is it if the fraction 16/12 divided by 3/12 ? Will it work?(4 votes)
- You can absolutely do this. When dividing fractions there are a few methods you can work out. The problem you gave would have an answer of 16/3.
Here is a khan academy video on working out a dividing fractions problem: https://www.khanacademy.org/math/arithmetic/fraction-arithmetic/arith-review-dividing-fractions/v/dividing-fractions-example&sa=U&ved=2ahUKEwj0i_rWzOjoAhVnknIEHa_cAtEQFjAAegQIAhAB&usg=AOvVaw2M_V5C7X7Fe5TxqTpMGmeP(2 votes)
- at 1.33 what is the colored boxes soppost to mean?(3 votes)
- He is visually showing how to subtract those fractions. There's 18 boxes and he colors in 8 of them to represent 8/18. Then he gets rid of 5 of them to show the subtraction. and how there is 3 boxes left, or 3/18 left.(4 votes)
- What about subtracting numbers with different denominators? Could you describe it carefully?(4 votes)
- You would find a common denominator. You do that by finding a number that is a factor of both of the numbers in the fraction. If I were you, I would look up a Khan Academy tutorial on it.(2 votes)
We're asked to subtract and simplify the answer, and we have 8/18 minus 5/18. So subtracting fractions is very similar to adding fractions. If we have the same denominator, the denominator in the difference is going to be the same as the denominators in the two numbers that we're subtracting, so it's going to be 18. And our numerator is going to be the difference between the numerators. So in this case, it is 8 minus 5, and this will be equal to 3 over 18, which is the answer, but it's not completely simplified, because both 3 and 18 are divisible by 3. So let's divide them both by 3. So you divide 3 by 3, you divide 18 by 3, and you get 3 divided by 3 is 1. 18 divided by 3 is 6, so you get 1/6. And just to see this visually, let me draw 18 parts. Let me draw 18 parts here. So it might be a little bit of a messy drawing. I'll try the best I can. So let me draw six in this direction. So that is three right there. We have another three, so that's six parts. And then let me split this into three columns. So there we go. We have 18 parts. Now 8/18 is equal to one, two, three, four, five, six, seven, eight. That's 8/18. And now we want to subtract five of the eighteenths, so we subtract one, two, three, four, five. Now, what do we have left over? Well, we have three of the eighteenths left over, so you have that right there. You have three of the eighteenths left over. Now, if you turn three of the eighteenths into one piece, how many of those bigger pieces do you have? This is one of those big pieces. Now, where are the other ones? Well, this is another big piece right here. This is another big piece right here, another one, another one, and another one. If you had 18 pieces and you merged three of the pieces into one, then you actually end up with only six pieces. You end up with six pieces. Hopefully, you see that each row is one of the pieces now, and the blue is exactly one of the six, so 3/18 is the same as 1/6.