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# Why can't you easily add or subtract fractions with different denominators?

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## Related Questions

###### Asked in Math and Arithmetic

### How do you add and subtract fractions that don't have common denominators?

Take for example 1/2 and 1/3. Since those denominators are
different, one must change them so they do have "common
denominators".
Most often, the easiest way of doing this is to simply multiply
the two denominators, and perform the same function on the top
numbers (the numerators).
So 2 times 3 is six, so 6 will now be the denominator of both
fractions.
As you changed the 2 into a 6 by multiplying by 3, you must
multiply the 1 above it by 3. This gives you the final first
fraction of 3/6.
The three in the second fraction was multiplied by 2 to get 6,
so you must multiply it's top number by 2. This gives you a final
fraction of 2/6 for the second fraction.
Now you are dealing with the fractions 3/6 and 2/6, and you can
easily add and subtract them.

###### Asked in Math and Arithmetic, Algebra, Percentages, Fractions, and Decimal Values

### What are fifteen facts or more about fractions?

You're only supposed to ask one question at at time but here we
go:-
1 Fractions are parts of whole numbers or integers
2 Fractions less than 1 are common fractions
3 Fractions greater than 1 are improper fractions
4 Fractions have denominators which are underneath their
numerators
5 Fractions are separated by a solidus line such as n/d
6 Fractions that are improper can be changed into mixed
numbers
7 Fractions can be changed into decimals
8 Fractions can be converted into percentages
9 Fractions are rational numbers
10 Fractions can not be derived from irrational numbers
11 Fractions need a LCD when adding or subtracting them
12 Fractions can be easily multiplied and divided
13 Fractions can be equivalent such as 2/3 = 4/6
14 Fractions can be simplified by finding their HCF
15 Fractions use prime numbers to find the LCM of different
denominators
16 Fractions were once used by the ancient Romans to a limited
extent

###### Asked in Factoring and Multiples

### What is the LCD of 2 4 5?

The LCD, or Lowest Common Denominator, is the smallest multiple
of each of the denominators of a set of fractions. So, assuming
that 2, 4 and 5 are denominators of fractions (1/2, 1/4 and 1/5,
for example), the LCD would be 20, because 20 is the lowest number
that 2, 4 and 5 multiply into. So, your new fractions would be
10/20, 5/20 and 4/20. The purpose of finding the LCD is to allow
for multiplying fractions together, or simply comparing them
easily.

###### Asked in Algebra, Math and Arithmetic

### What is the sum of three dividide by fifteen plus tweleve dividide by eighteen is?

3/15 + 12/18 = ???
To add (or subtract) fractions a common denominator needs to be
found. This is usually the lowest common multiple of the present
denominators but if this cannot be easily calculated use the
product of the present denominators. The LCM of 15 & 18 is
90.
3/15 = 3x6/15x6 = 18/90 : 12/18 = 12x5/18x5 = 60/90
Then 18/90 + 60/90 = 18+60/90 = 78/90 which can be simplified to
39/45

###### Asked in Math and Arithmetic

### How do you change a common multiple into a denominator to make it an equivalent fraction?

To use common multiples to make equivalent fractions, use this
example:
change 2/3 and 5/9 into equivalent fractions.
the common multiple is: 9
change both denominators to nine (keep in mind: whatever you do
to the denominator you must do to the numerator)
you get 2/3 = 3 x 3 = 9 so 2 x 3 = 6
so 2/3 = 6/9
you can now compare 6/9 and 5/9. you can add and subtract
easily.

###### Asked in Math and Arithmetic

### Do you use notation to compare and order fractions?

You can use the same notation and ordering for fractions as you
do integers. The difficulty with fractions is that in most cases
you need to find eqivalent denominators to see how they rank. Ie.
If I said order for smallest to largest 2/3, 1/6, 72/96 and 24/48.
It would be difficult without finding some similar base (is
2/3>72/96?). Instead if you conver them into a common base...
8/12, 2/12, 9/12, 6/12. Now you can easily order and/or compare the
fractions.

###### Asked in Rhetorical Questions, Education, How To

### How to explan fractions to a sixth grade student?

Use visual aids that you can draw and/or cut yourself. Use
objects the child recognizes such as sliced pizza, cake, pie. Also
use squares, rectangles and other figures that are easy to "slice"
into several pieces. Each piece will be a fraction that can be
easily seen and understood. You can use the pieces to add and
subtract the fractions.

###### Asked in Loans, Math and Arithmetic

### How do you subtract two and two thirds from eleven and one half?

You need to do two changes before you can do the actual
subtraction.
One, since 2/3 is more than 1/2, you have to write 11 1/2 as 10
3/2. That is, you add 2/2 to the fractional part, and, to
compensate, you subtract the equivalent (2/2 = 1) from the integer
part.
Two, you have to convert the fractions to a common
denominator.
Once you have done these two changes, you can easily subtract.
(Subtract the integer part and the fractional part separately.)

###### Asked in Math and Arithmetic, Algebra, Numbers

### What are a score or more facts about friendly fractions?

1 Fractions are parts of whole numbers or integers
2 Fractions have numerators above their denominators
3 Fractions have a solidus line that separates numerator from
denominator
4 Fractions can be common as for example: 3/4
5 Fractions can be improper or 'top heavy' as for example:
22/7
6 Fractions can form part of a mixed number as for example: 3
and 1/7
7 Fractions can be converted into percentages as for example:
1/2 = 50%
8 Fractions can be converted into decimals as for example: 3/4 =
0.75
9 Fractions can be equivalent: 5/8 = 10/16
10 Fractions need a common denominator when added or
subtracted
11 Fractions can easily be multiplied or divided
12 Fractions are rational numbers
13 Fractions can never ever be irrational numbers
14 Fractions can be turned into improper fractions from mixed
numbers
15 Fractions are used in algebra and trigonometry
16 Fractions are used in converting Celsius into Fahrenheit
17 Fractions are turned into decimals by dividing denominator
into numerator
18 Fractions must be eliminated when solving algebra
equations
19 Fractions can be the solutions of quadratic equations
20 Fractions can be changed into scientific notation: 1/5000 =
2.0*10^-4
21 Fractions are in their lowest terms when their HCF is one

###### Asked in Math and Arithmetic, Algebra, Numbers

### What are a score or more fundamental facts about fractions?

1 Fractions are parts of whole numbers or integers
2 Fractions have numerators placed above denominators
3 Fractions have a solidus line that separates numerator from
the denominator
4 Fractions can be common as for example 3/4
5 Fractions can be improper or 'top heavy' as for example
22/7
6 Fractions can form part of a mixed number as for example 3 and
1/7
7 Fractions can be converted into percentages: 3/4 = 75%
8 Fractions can be converted into decimals: 1/2 = 0.5
9 Fractions can be equivalent: 5/8 = 10/16
10 Fractions need a common denominator when added or
subtracted
11 Fractions can easily be multiplied and divided
12 Fractions are rational numbers
13 Fractions can never ever be irrational numbers
14 Fractions are turned into decimals by dividing numerator by
denominator
15 Fractions can be turned into improper fractions from mixed
numbers
16 Fractions are used in algebra and trigonometry
17 Fractions are used when converting Centigrade to
Fahrenheit
18 Fractions are best eliminated before solving equations
19 Fractions can be the solutions of quadratic equations
20 Fractions can be changed into scientific notation: 1/5000 =
2.0*10-4
21 Fractions are in their lowest terms when their HCF is one
22 Fractions were once used by the ancient Romans to a limited
extent as for example the numeral of S represents 1/2
QED

###### Asked in Math and Arithmetic, Algebra, Percentages, Fractions, and Decimal Values

### What are a score or more mathematical facts about fractions?

1 Fractions are parts of whole numbers or integers
2 Fractions have numerators above their denominators
3 Fractions have a solidus line that separates the numerator
from the denominator
4 Fractions can be common as for example 3/4
5 Fractions can be improper or 'top heavy' as for example
22/7
6 Fractions can form part of a mixed number as for example 3 and
1/7
7 Fractions can be converted into percentages as for example 1/2
= 50%
8 Fractions can be converted into decimals as for example 3/4 =
0.75
9 Fractions can be equivalent as for example 5/8 = 10/16
10 Fractions need a common denominator when adding or
subtracting them
11 Fractions can easily be multiplied or divided without a
common denominator
12 Fractions are rational numbers
13 Fractions can never ever be irrational numbers
14 Fractions are turned into decimals by dividing the
denominator into the numerator
15 Fractions can be turned into improper fractions from mixed
numbers
16 Fractions are used in algebra and trigonometry
17 Fractions are used in converting Celsius into Fahrenheit
18 Fractions must first be eliminated when solving equations
19 Fractions can be the solutions of quadratic equations
20 Fractions can be turned into scientific notation: 1/5000 =
2.0*10^-4
21 Fractions are in their lowest therms when their HCF is 1
22 Fraction derives from a Latin word meaning to break apart
23 Fractions were used by the ancient Romans to a limited
extent
24 Fractions can be the solutions of simultaneous equations
25 Fractions can be the x and y coordinates on the Cartesian
plane

###### Asked in Math and Arithmetic

### How does a nurse use decimals?

A strong working understanding of fractions and decimals is
essential for nurses. They must be familiar enough with fractions
and decimals to quickly and accurately divide, multiply, add and
subtract dosages as well as convert fractions to decimals and vice
versa. Conceptual understating of fractions and decimals is
essential since half doses, extra doses and time-delayed dosages
must be calculated correctly. Nurses also need to know how to
convert fractions and decimals to percentages in order to explain
medication instructions accurately and easily to their
patients.
Read more about how math is related to nursing at the link I
provided below.

###### Asked in Math and Arithmetic, Percentages, Fractions, and Decimal Values

### How do you know that a fraction is closest to one half?

In order to compare two fractions, you have to convert them so
that they have the same denominator, which is to say, they are the
same kind of fraction, whether that is thirds, quarters, fifths,
etc. Let's say that I want to compare 2/9 with 1/5. I can make them
both into 45ths. Multiply the 2/9 by 5/5 and you get 10/45.
Multiply the 1/5 by 9/9 and you get 9/45. Now you can compare,
because 10/45 is obvious 1/45 larger than 9/45. In the example
given, since both fractions are less than a half, the larger one is
closer to a half. If I had two fractions that were both larger than
a half, then the smaller one is closer to a half. What if I have a
fraction that is larger than a half and another fraction that is
smaller than a half, and I want to know which is closer to a half?
I would have to convert all 3 fractions (half is also a fraction)
so that they have common denominators, then I can easily subtract a
candidate fraction from a half, or subtract a half from it, and see
which gives the biggest difference.

###### Asked in Algebra

### What is 4cm - 28mm?

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