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Multiplying mixed numbers

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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  • blobby green style avatar for user Arbaaz Ibrahim
    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?
    (36 votes)
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    • hopper cool style avatar for user Seed Something
      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!
      (45 votes)
  • blobby green style avatar for user aliciaafinney
    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?
    (10 votes)
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    • hopper cool style avatar for user Seed Something
      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
      =
      7/436/5
      =
      Ok! this is where it diverges, with and without cross cancel simplifying…

      Original Fractions
      (not cross-cancelled)
      =
      7/436/5
      =
      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!
      (10 votes)
  • duskpin ultimate style avatar for user Jennifer Kleinhans
    What if you can't divide any of the Numerator's or Denominator's by anything?
    (7 votes)
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  • marcimus pink style avatar for user Cheyla Diorr
    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.
    (10 votes)
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  • starky ultimate style avatar for user Misia
    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 .
    (10 votes)
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  • starky tree style avatar for user Joy14
    What if we don't have a remainder in the progress of making improper fraction to mixed number?
    (7 votes)
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  • hopper cool style avatar for user behemoth
    what is the opposite of 0.93
    (4 votes)
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  • blobby green style avatar for user 20202219
    Yeah learned all of this in 5th why are we learning this now cause I'm in 7th
    (7 votes)
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  • stelly blue style avatar for user cherry <3
    i get this but he didnt explain what to do with a whole number?
    (4 votes)
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    • female robot ada style avatar for user Eloquent_Reader
      When dealing with whole numbers in an equation, you can easily convert them into fractions by placing them over 1. For instance, if you have the equation 1/3 x 5, you can convert 5 into a fraction by writing it as 5/1. This does not change the value of 5 but makes it easier to multiply. We can now proceed to multiply 1/3 by 5/1, which equals 5/3. Simplifying 5/3 gives us 1 2/3. I hope this explanation has been helpful in answering your question.

      1/3 x 5 = ?
      5 = 5/1
      1/3 x 5/1 = 5/3
      5/3 = 1 2/3
      1/3 x 5 = 1 2/3
      (7 votes)
  • aqualine tree style avatar for user if11106
    is there any other videos for this question?
    (7 votes)
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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.