Fractions

# Adding and subtracting fractions with unlike denominators

## How to add fractions that have different denominators

To solve this problem, we need to find the least common multiple to get at the common denominator. Can you help? We bet you can!
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## How to add fractions that have different denominators

Discussion and questions for this video
why does the denominator have to be the same as the other denominator
First understand that x/y means "x" parts of a "whole" which is divided into "y" parts..
Now an example:
Suppose a class has 100 students, for a particular task 1/4 students were selected, and other 1/5 students were selected for a 2nd task. Find how many students have been selected from the class in all. [(1/4 + 1/5) * 100] = ?
Solution,
1/4 of the class will be 1/4 * 100 = 25 ; 1/5 of the class will be 1/5 * 100 = 20
25+20 = 45; the answer will be 45 students
But if we directly add both the fractions we get 1/4 + 1/5 = 2/9 ; 2/9 * 100 = 22.22 students; which is not the correct answer.
To solve this problem we need to first find a LCM (Lowest Common Multiple) of the denominators (4 & 5) which is 20.
Now 1/ 4 becomes 5/20 and 1/5 becomes 4/20;
So 1/4 + 1/5 is now = 5/20 + 4/20 = 9/20
9/20 * 100 = "45" the correct number of students.
Thus finding LCM means finding a common number of parts; which also keep the ratio same; 1/4 = 5/ 20 and 1/5 = 4/20.
Hope this helps..
What does it mean to simplify the problem?
Simplifying a fraction just means putting it into the simplest form. For example, 3/6 can be simplified to just 1/2. They are the same thing, but 1/2 is simplified.
Why do you have to get the common denominator, can't you just add?
No. If the denominators are unlike you are comparing two completely different things. Think of fractions like pizza. The total number of slices is the denominator. What happens when you increase the total number of slices while keeping the pizza the same size? The slices get smaller. A pizza with 4 slices will have smaller slices than a pizza with just 3 slices. So if you don't keep the denominator (or number of slices) the same your definition of what constitutes a slice (or fraction) changes and your answer no longer makes sense. It maybe easier to comprehend if you see it visualized, checkout http://mathmistakes.info/facts/AlgebraFacts/learn/fr/add.html
why do we use the same denominators, when we are doing the adding when we have a different denominators.
i just guess its the rules
1 Vote
1 Comment
Opps. I don't get it
it wont let me do practice problems
Does anyone get this?
ok then i understand😍
What do you mean?
can you just add both of the numbers of in top and bottom da
If you understand about adding the fractions together, and changing the denominators, see the video on subtracting fractions. (http://www.khanacademy.org/math/arithmetic/fractions/v/subtracting--fractions). There isn't one specifically on subtracting fractions with unlike denominators.
1 Comment
Why does the denominator have to be the same as the other denominator?
This is the tip of the actual people that understand this "adding or subtracting" it has different "rules" of what to do.... is all about process and know when to do the operations.
at the end do you have to divide the numbers
If you're talking about 3:45 then no.
I already knew how to do this, but it gave me a couple pointers.
if 12x3 is 36 then why did he not do 9x3
I don't get this it is hard can anybody help me?
1 Comment
it is quite simple if you ask me all you have do is this Whenever you add fractions with unlike denominators, you must make the denominators of the same value. In this example, the easiest approach is to multiply 8 x 3 to get 24. Here, you multiply by 3 to get . Then you multiply the second equation by 8 to get . Note that both addends have 24 as a denominator. Add in the same manner as with the unit Adding fractions with like denominators.
How do You do tris
The formula is (HOCH^2)^3 CNH^2 , but this is math not chemistry.
3/4 + 1/2=answer in simplest terms
In order to add fractions, the denominators must be the same.
The lowest common denominator or multiple (LCD or LCM) is 4.
Whatever we multiply the denominator by, we must multiply the numerator by.
Therefore, 1/2 = 2/4.
So, 3/4 + 2/4 = 5/4 as an improper fraction or as 1 1/4 as a mixed number.
Mrs Dyer baked 5 1/2 batches of brownies. She needed 3/4 cups of vegetable oil for each batch she makes. How much vegetable oil will she bake her brownies?
math is good i love math ( ͡° ͜ʖ ͡°)
how would you break own 3/5 + 1/11
3/5 + 1/11 = 38/55.
why do you need to multiply the numerator when you multiply the denominator to add or subtract fractions?
Good question. The answer is that if you didn't then you would change the value of the fraction. Like 2/3. If you multiplied both of them by 2, you would have 4/6. Both equal 0.66. If you multiplied the bottom only, then it would be 2/6 which is equal to 0.33. It would change the value.
if there was a like button, as skydoesminecraft would say "Slap that like button!"
math is good i love math ( ͡° ͜ʖ ͡°)
can both fractions be negativs
Right Patrick Crandon!
why do you have to find a common denominator?
math is good i love math ( ͡° ͜ʖ ͡°)
I like fractions?
1 Comment
This should be in the tip/thanks section. (I am not a moderator, but I'm pretty sure this is against the rules as irrelevant)
Hello? fractons!
Fractions are so fun!
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1 Comment
math is good i love math ( ͡° ͜ʖ ͡°)
what to do with 7/6
math is good i love math ( ͡° ͜ʖ ͡°)
This video needs another one
wow it really helped me ;) ive got a question ..um how do you do this but is there a easy way?
I don't get it at all. He is doing it right but, I don't get the part, I mean all the parts. The only part I get is when you got to times it to find the same number. Anyone can help me pleas do.......
I really think you should do more, A LOT more fractions!
Sometimes I dont get it when you have a whole number
math is good i love math ( ͡° ͜ʖ ͡°)
What does the ✔ Mark means
Sometimes it means wrong :p
So what if there is no improper fraction. Will you still have to simplify it?? Like for example my daughter has a math worksheet and it says 3/8 + 1/4 and she got 5/8 for the answer but she is trying to figure out a way to simplify it even though it's not an improper fraction
math is good i love math ( ͡° ͜ʖ ͡°)
Can we do another step like this:
3/8+5/9=(3*9)+(5*8)/72=27+40/72=67/72
Yes, that method works perfectly.