Main content

### Course: 4th grade > Unit 9

Lesson 3: Multiplying whole numbers and mixed numbers# Multiplying mixed numbers by whole numbers

Sal uses repeated addition to help multiply a whole number by a mixed number.

## Want to join the conversation?

- can anyone explain how 2 1/4 became

8/4 + 1/4??(23 votes)- Kinda stupid of me replying but I don't understand either but I think 2 1/4 becomes 8/4 because 2x4=8 or maybe 2x1=2 I don't really know so don't judge me :((10 votes)

- First of all, 9 X 4 IS 36 NOT 27, 27 IS 9 X 3(11 votes)
- yes that is correct even tho i dont really know much about multipulcation(11 votes)

- where did you get the number (8/4 + 1/4) from(10 votes)
- This is what he did:

2 1/4 (mixed number)

2 + 1/4 (2 wholes, 1/4 part)

2*4/4 + 1/4 (4/4 = 1 so there's no change to the expression)

8/4 + 1/4

9/4

Hope it helps anyone(7 votes)

- 2 1/4x3

2x1=2

2x3=6, I'm I doing it wrong?

I don't understand.(6 votes)- Exe, to make it easier, you should convert 2 1/4 into an improper fraction

The improper version of 2 1/4 is 9/4.

BUT, 9/4 x 3, can be made even EASIER.

All whole numbers, positive or negative, have a fraction version.

All of them are 'something'/1.

3 can be interpreted as 3/1.

So, 9/4 x 3/1 is 27/4.

27/4's mixed number form is 6 3/4.

Hope this helps, Exe.(5 votes)

- 2:23_ 6 + 3/4 is just 6 3/4? I thought we had to find the lowest common denom. to add fractions, not just tack 6 onto 3/4. what am I missing?(7 votes)
- Is there a quicker way of doing it?(5 votes)
- how do I know how to approach the math problems? For example in one math equation it says to 3 5/6 times two. How do I know wether to simplify the equation or not to? In different scenarios it is hard to do if you don't understand which method to use.(5 votes)
- I don't understand at all!(3 votes)
- Remember, multiply the whole numbers first, multiply the numerator and the whole number together to get an improper fraction. Change the improper fraction into a mixed number and add. Example: 7 6/7 x 5. 7x5=35, 5x6=30 30 divided by 7 = 4 r2. Now you want to take your remainder and make it a fraction. In this case, 2/7. Now you add the whole numbers and the fraction together to get your product. In this case, 39 2/7.(4 votes)

- I can't submit! 😶(4 votes)
- refresh the page or try using the enter key.(1 vote)

- i always watch this video but then i get a 0 on the excersises lol sal plz explain more(4 votes)

## Video transcript

- [Instructor] Let's
see if we can figure out what 2 1/4 times three is. Pause this video and see
if you can work that out. All right, now there's different ways that we could approach this. One way to approach this is to recognize that if I multiply anything times three, that means that I'm taking
three of these things and adding them together. So, this is going to be the same thing as 2 1/4 plus 2 1/4 plus 2 1/4. Now, I could also view
this as each of these 2 1/4 is two plus 1/4 plus two
plus 1/4 plus two plus 1/4. All I did is I broke up each of the 2 1/4 into a two plus 1/4. And then what I could do is I could add the whole number parts, two plus two plus two is
going to be equal to six. And then, if I were to
take 1/4 plus 1/4 plus 1/4, how many fourths do I now have? I have 3/4, so it's gonna be six plus 3/4, or I could write this as 6 3/4. Now, another way that we
could have approached this is we could've rewritten 2 1/4. I could have rewritten that as
the same thing as two is 8/4, 8/4 is the same thing as two, plus 1/4 and then I'm multiplying that whole thing times three. And so, this is going to be the same thing as 9/4, 8/4 plus 1/4 is 9/4, times three. And so, we could say, "Hey,
that's just going to be equal "to 9/4 plus 9/4 plus 9/4,
which would be equal to what?" Well, I have, this is going to be nine
plus nine plus nine fourths, which is going to be equal to 27/4. Now, you might be saying, "Hey, these look like they are different", but you could check this. Six is equal to how many fourths? So, six is equal to 24/4, 24/4 plus 3/4, which is
exactly equal to 27/4. And we are done.