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## 5th grade

### Unit 6: Lesson 4

Multiplying mixed numbers# Multiplying mixed numbers

CCSS.Math:

Multiplying mixed numbers is similar to multiplying whole numbers, except that you have to account for the fractional parts as well. By converting mixed numbers into improper fractions, you can multiply the two numbers together in a straightforward way. Once you have the product as an improper fraction, you can convert it back into a mixed number. Created by Sal Khan and Monterey Institute for Technology and Education.

## Want to join the conversation?

- When we have one mixed number and one whole number, why do we only multiply the numerator; for example; 9 x 1 1/12 = 9x13 /12, why can't we do 9 x 13/ 9x12?(0 votes)
- →
**1st question**:

'When we have one mixed number and one whole number, why do we only multiply the numerators?'

•When calculating a**Whole Number × a Fraction**it can**appear like****only the numerators are multiplied, (but the denominators are too).***The unseen denominator math is:*

(1 × other denominator), because all whole numbers have a denominator of one*,*.

so the calculation always equals the other denominator

So even without knowing why, by default we still get the correct denominator.

→**2nd question**:

'9 x 1 1/12 = 9x13/12

Why can't we do 9x13/9x12?'

•**We don't multiply the Whole Number to both the numerator and denominator**, because it mimics a Multiplicative Identity Fraction 9/9 = 1, (so ×1, no longer ×9).

So it doesn't answer to 9 × 1 1/12,**it results in a wrong value.**…

★Deeper look into both answers

First we transform the Mixed Number value into an Improper Fraction, (denominator × whole number + numerator, keep denominator), ex…

Nine times one and one twelfths.

=

9 × 1 1/12

=

9 × (**12 × 1 +1**)/12 ←transforming

=**9 × 13/12**).

= …

To multiply fractions:

(numerator × numerator), and

(denominator × denominator,**A Whole Number's denominator always equals one****so that makes the multiplication**:**always**

(**1 × other denominator**).

Therefore the whole number 9 has a denominator of one!**So the calculation is always the same, it's considered 'understood', so the following denominator math often isn't shown, except when learning it**:

9 × 13/12

=

9/**1**× 13/12 ←showing denominators

=

(9 × 13)/(**1 × 12**) ←often not shown

=

(9 × 13)/**12**, the 'other' denominator.**So mathematically the denominators are multiplied too, it's presumed 'known' to have occurred**, we just don't bother writing it out because it always results in the denominator not equal to 1

→**Question 2**

'9 x 1 1/12 = 9x13/12

Why can't we do 9x13/9x12?'

•**We can't do: 9×13/9×12,**.

9 ×numerator/9 ×denominator,

because it would be a miscalculation, and equivalent to: 9/9 × 13/12

9/9 is a Multiplicative Identity Fraction: the same numerator and denominator is equal to 1.

**so it won't solve: 9 × 13/12**

instead it's…

(**9 × 13**)/(**9 × 12**)

=

117/108

=

Simplify with GCF: 9

=**13/12 ←wrong value**

It's multiplying by a fraction that equals one, so after we simplify, we're back to 13/12 again.

★**Complete calculations for:**

nine times thirteen twelfths

=

9 × 13/12

=

(9 × 13)/(1 × 12) ←often unseen

=

(**9 × 13**)/**12**

=

117/12

simplify with GCF 3

=**39/4**←correct value 🥳

=

9 3/4 ←mixed number form

=

9.75 ←decimal form

(≧▽≦) Hope this helps someone!(21 votes)

- What if you can't divide any of the Numerator's or Denominator's by anything?(7 votes)
- Good question Jennifer,

if you cant divide either the numerator or the denominator it will stay the same number,

10/10 = 10(/)2/10(/) = 5/5- you cannot

simplify 5/5 so it will stay 5/5.

((/) is division)

Hope this answers your question(13 votes)

- idk if someone asked but When you multiply a whole number by a fraction, you only multiply and whole number by the numerator. It's because a whole number is a whole, (or wholes), which makes it unnecessary to multiply it with the denominator. Like for example, 9 = 9/1, correct? So when we do 9/1 * 1 1/2, the denominator is not effected.(7 votes)
- Correct! you have the right idea!

It is because, so when it's multiplied the result will always be the other fraction's denominator.**a Whole Number's denominator is one**

Which is*why we can immediately multiply the whole number to the numerator of a fraction*, and still have the correct denominator value.

★In Comments:

•Example Walkthrough and

•**Different Method Solution**(1 vote)

- Can you use the same method to multiply 5×6 1/3?(3 votes)
- what is the opposite of 0.93(0 votes)
- To get an opposite of a number, just change the sign. If it is a negative, make it positive. If it is positive, make it a negative(6 votes)

- can anyone explain to me beacuse i am confused(1 vote)
- Sure! Multiplying a mixed number is easy once you have figured it out. All you have to do is make the mixed number an improper fraction, and then multiply them. Here's an example.

Let's say you were asked to multiply 1 1/3 by 1 1/2. First, you would want to make these mixed numbers**improper fractions**.

1 1/3 = 4/3

1 1/2 = 3/2

Now they are much easier to multiply. You can now rewrite the question to this:

4/3 * 3/2 (* is a multiplying symbol.)

First, multiply the numerators:

4 * 3 = 12

Then the denominators:

3 * 2 = 6

Now you have 12/6 as your final answer, or 2. Hope this helped you. :)(5 votes)

- I have a question how do you do 6/11 to 1 1/6

I’m having trouble getting the answer plz can you teach us 😊 this week and you helped for most probs.(2 votes) - What is an Improper Fraction?(3 votes)
- Hello, Leilani!

To answer your question, Improper Fractions are fractions when the numerator (ex.**1**/6), is larger than the denominator. Here is what an improper fraction would look like:

9/2

10/5

Hope this helps someone :>(1 vote)

- Is there there another way to multiply fractions?(2 votes)
- how would you do 6 3/4 times 3 3/4(2 votes)
- You can make both numbers improper fractions(27/4, 15/4) and multiply the numerator by the numerator and the denominator by the denominator to get the answer.(2 votes)

## Video transcript

Multiply 1 and 3/4
times 7 and 1/5. Simplify your answer and write
it as a mixed fraction. So the first thing we want to
do is rewrite each of these mixed numbers as improper
fractions. It's very difficult, or at least
it's not easy for me, to directly multiply
mixed numbers. One can do it, but it's much
easier if you just make them improper fractions. So let's convert each of them. So 1 and 3/4 is equal to-- it's
still going to be over 4, so you're still going to have
the same denominator, but your numerator as an improper
fraction is going to be 4 times 1 plus 3. And the reason why this makes
sense is 1 is 4/4, or 1 is 4 times 1 fourths, right? 1 is the same thing as 4/4, and
then you have three more fourths, so 4/4 plus 3/4
will give you 7/4. So that's the same thing
as 1 and 3/4. Now, let's do 7 and 1/5. Same exact process. We're going to still be talking
in terms of fifths. That's going to be
the denominator. You take 5 times 7, because
think about it. 7 is the same thing as 35/5. So you take 5 times 7 plus this
numerator right here. So 7 is 35/5, then you have
one more fifth, so you're going to have 35 plus 1,
which is equal to 36/5. So this product is the exact
same thing as taking the product of 7/4 times 36/5. And we could multiply
it out right now. Take the 7 times 36 as our new
numerator, 4 times 5 as our new denominator, but that'll
give us large numbers. I can't multiply 7 and
36 in my head, or I can't do it too easily. So let's see if we can
simplify this first. Both our numerator and our
denominator have numbers that are divisible by 4, so let's
divide both the numerator and the denominator by 4. So in the numerator, we can
divide the 36 by 4 and get 9. If you divide something in the
numerator by 4, you need to divide something in the
denominator by 4, and the 4 is the obvious guy, so 4
divided by 4 is 1. So now this becomes 7 times 9,
and what's the 7 times 9? It's 63, over 1 times 5. So now we have our answer as an
improper fraction, but they want it as a mixed number
or as a mixed fraction. So what are 63/5? So to figure that out-- let me
pick a nice color here-- we take 5 into 63. 5 goes into 6 one time. 1 times 5 is 5. You subtract. 6 minus 5 is 1. Bring down the 3. 5 goes into 13 two times. And you could have immediately
said 5 goes into 63 twelve times, but this way, at
least to me, it's a little bit more obvious. And then 2 times 5 is 12,
and then we have sorry! 2 times 5 is 10. That tells you not to
switch gears in the middle of a math problem. 2 times 5 is 10, and then you
subtract, and you have a remainder of 3. So 63/5 is the same thing as 12
wholes and 3 left over, or 3/5 left over. And if you wanted to go back
from this to that, just think: 12 is the same thing as
60 fifths, or 60/5. 60/5 plus 3/5 is 63/5,
so these two things are the same thing. These two things
are equivalent. This is as an improper
fraction. This is as a mixed number
or a mixed fraction. But this is our answer right
there: 12 and 3/5.