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## 5th grade

### Course: 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?(32 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!(25 votes)

- What if you can't divide any of the Numerator's or Denominator's by anything?(6 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(14 votes)

- When simplifying the fraction prior to multiplying it, why is it that you can change the numerator and denominator of opposite fractions? My understanding of simplification is that the purpose is to create a more wieldy, but equivalent, number, and my confusion is that when we do this simplification we get 7/1 (= to 7) and 9/5 (= to 1 4/5) which are not equivalent at all to 1 3/4 (7/4) and 7 1/5 (36/5) respectively. How does this give us the answer? Haven't we just arbitrarily created a different number, that will not give us the same answer as the numbers we've started with?(6 votes)
- Great Question!
is it the same answer?**How and Why**

(since the*individual fractions*before and after*are not equal*)

7/4 • 36/5 ←original values

=

7/1 • 9/5 ←cross simplified

first fractions:

7/4 ≠ 7/1

and second fractions:

36/5 ≠ 9/5

So…

★**Cross Cancellation simplifies**at a easier time, by*before*the fraction multiplication*the same GCF if used after multiplication*.

Given:

One and three fourths × Seven and one fifths

1 3/4 • 7 1/5

=

Transform Mixed Numbers to Fractions: (denominator × whole number + numerator, keep denominator)

(**4 · 1 + 3**)/4 • (**5 · 7 + 1**)/5

=**7/4**•**36/5**

=

Ok! this is where it diverges, with and without cross cancel simplifying…

★**Original Fractions**

(not cross-cancelled)

=**7/4**•**36/5**

=**252/20**

=

Simply,**GCF 4**←answer 🥳

(252 ÷4)/(20 ÷4)

=

63/5

=

12.6

★**Cross Cancel/Simplify**

(In fraction multiplication, a numerator and denominator*of opposite fractions divided by a common factor*.)

=**7/4 • 36/5**

1st denominator to 2nd numerator**GCF 4**

=

7/(**4 ÷ 4**) • (**36 ÷4**)/5

=

7/1 • 9/5

=**63/5**←same value 🥳

=

12.6

It works because…

★**Cross Cancelling is a short cut for longer Arithmetic processes**…

•In division:**If we write out each step****,the same simplification chance is available through Division**:**7/4 • 36/5**←🥳

=

(7 · 36)/(4 · 5) ←division chance

=

(7 · 9)/(1 · 5) ←becomes

=

63/5

and the division chance works because…

★In*multiplication*:**The original fractions are**…*rewritten, and rearranged*

1st fraction:

7/4 = 7 • 1/4

2nd fraction:

36/5 = 36 • 1/5

7/4 • 36/5

=

7 • 1/4 • 36 • 1/5

=

Multiplication is Commutative, (interchangeable, rearrangeable), we can swap the order of factors…

swap 7 with 36

=

36 • 1/4 • 7 • 1/5

=

36/4 • 7/5

=

9 • 7/5

=

63/5 ←🥳

=

12.6 ←decimal

=

12 3/5 ←mixed number

so…

★**Cross Cancelation is a**.*shortcut to less arithmetic steps, and simpler values*, each step of overall expression is equal,*so we still get the same answer*

(≧▽≦) I hope this helps someone!(2 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.(6 votes)
- But if you multiply a whole number by a mixed number as your example, you have to be careful, it is not as simple as multiplying numerators and denominators.(4 votes)

- What if we don't have a remainder in the progress of making improper fraction to mixed number?(5 votes)
- If you have zero for a remainder, the fractional part of the mixed number has a zero in the numerator. So if you mixed number is something like 5 0/7 that is equal to simply 5. So, No Remainder = No Fractional part of a Mixed Number.(2 votes)

- what is the opposite of 0.93(2 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)

- Upvote this and I will upvote you! If it reaches the top, I will be grateful and tell my friends to upvote you as well. ! ' Yavdeto Yatpo ' yato ' Yaby Yastro ' yayruo yatnru , Yayeigna .(5 votes)
- Can you use the same method to multiply 5×6 1/3?(3 votes)
- 0:42Why does Sal write 4.1?(2 votes)
- The dot is a raised dot that represents multiplication. It is not a decimal point.

1 3/4 = (1x4+3)/4 = (4+3)/4 = 7/4(3 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.