Question

how do you find the least common denominator?

Answer 1

Fractions with Different Denominators

You can't add fractions with different denominators:
1/3 + 1/6 = ?
1/3 1/6 1 1

So what do you do? How can they be added?

Answer: You need to make the denominators the same.
Common Denominator

But what should the new denominator be?

One simple answer is to multiply the current denominators together:

3 × 6 = 18

So instead of having 3 or 6 slices, we will make both of them have 18 slices.

The pizzas now look like this (I will show calculations later):
6/18 + 3/18 = 9/18
1/3 1/6 1 1

(Read more about Common Denominators.)
Least Common Denominator

That is all fine, but 18 is a lot of slices ... can you do it with fewer slices?

Here is how to find out:
1/3 List the multiples of 3: 3, 6, 9, 12, 15, 18, 21, ...
1/6 List the multiples 6: 6, 12, 18, 24, ...

Then find the smallest number that is the same
multiples of 3: 3, 6, 9, 12, 15, 18, 21, ...
multiples 6: 6, 12, 18, 24, ...

The answer is 6, and that is the Least Common Denominator.

So let us try using it:
2/6 + 1/6 = 3/6
2/6 1/6 3/6 1

And that is what the Least Common Denominator is all about.

It lets you add (or subtract) fractions using the least number of slices.
What Did We Do?

The trick was to list the multiples of each denominator, then find the Least Common Multiple

In the previous example the Least Common Multiple of 3 and 6 was 6.

In other words the Least Common Denominator of 1/3 and 1/6 is 6.

Here are the steps to follow:


* Find the Least Common Multiple of the denominators (which is called the Least Common Denominator).
* Change each fraction (using equivalent fractions) to make their denominators the same as the least common denominator
* Then you add (or subtract) the fractions, as you wish!


Example: What is 1/6 + 7/15 ?

The Denominators are 6 and 15:
multiples of 6: 6, 12, 18, 24, 30, 36, ...
multiples 15: 15, 30, 45, 60, ...

So the Least Common Multiple of 6 and 15 is 30.



Now let's try to make the denominators the same.

Note: what you do to the bottom of the fraction,
you must also do to the top

When you multiply 6 × 5 you get 30, and when you multiply 15 × 2 you also get 30:
× 5
1 = 5

6 30
× 5
and
× 2
7 = 14

15 30
× 2



Now we can easily do the addition by adding the top numbers:

5/30 + 14/30 = 19/30