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5th grade
Unit 6: Lesson 4
Multiplying mixed numbersMultiplying 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.
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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?(30 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
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, the 'other' denominator.
→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)
- 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!
How and Why is it the same answer?
(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 before the fraction multiplication at a easier time, by 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
(252 ÷4)/(20 ÷4)
=
63/5 ←answer 🥳
=
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!(3 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 a Whole Number's denominator is one, so when it's multiplied the result will always be the other fraction's denominator.
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)
- what is the opposite of 0.93(3 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 you use the same method to multiply 5×6 1/3?(3 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)
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.