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Course: Class 7 (Old)>Unit 1

Lesson 3: Properties of multiplication

Properties and patterns for multiplication

Sal changes the order of numbers or decomposes numbers to simplify multiplication problems.    Created by Sal Khan.

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• why arent you following the order of operations? paranthesis first right?
(39 votes)
• I assume you are referring to about in the video where Sal changes 4*(5+2) into 4(5)+4(2).
Sal is using the distributive property. It distributes multiplication across addition or subtraction. It is an acceptable way of eliminating parentheses.
Hope this helps.
(44 votes)
• how would you solve (6x4)x5
can you solve that or no because the 5 should be in the front
(19 votes)
• You would solve that still as normal, but follow the order of operations(Parentheses, Exponents, Multiplication & Division, Addition & Subtraction) and do (6x4) first, then multiply that by 5. You could rearrange the equation as 5x(6x4) and the answer would still be the same, because of the Communicative Property. You would always do the expression within the parentheses first because that's how the order of operations goes. With these numbers though, you can rearrange the numbers however you want(whichever one makes it look the simplest to you) and the answer would always be the same, just make sure the operators stay the same(i.e don't turn the multiplication into division or subtraction).
(39 votes)
• how many properties of multiplication are there?
(7 votes)
• There are four properties involving multiplication that will help make problems easier to solve. They are the commutative, associative, multiplicative identity and distributive properties.

Commutative property: When two numbers are multiplied together, the product is the same regardless of the order of the multiplicands. For example 4 * 2 = 2 * 4

Associative Property: When three or more numbers are multiplied, the product is the same regardless of the grouping of the factors. For example (2 * 3) * 4 = 2 * (3 * 4)

Multiplicative Identity Property: The product of any number and one is that number. For example 5 * 1 = 5.

Distributive property: The sum of two numbers times a third number is equal to the sum of each addend times the third number. For example 4 * (6 + 3) = 4*6 + 4*3
(16 votes)
• How can I remember the Order of Operations
(4 votes)
• I learned order of operations with the phrase "Please Excuse My Dear Aunt Sally". The "P" means to solve the parentheses first, if there are any. "E" is the second step: solve exponents. "M" and "D" means to solve multiplication and or division. Be sure to solve from left to right! Lastly, "A" and "S" means to solve addition and subtraction, moving from left to right.
For example, (4+3) + 5^2 x 7 - 8 would be solved by PEMDAS:
Step 1. Solve parentheses. 7 + 5^2 x 7 - 8
Step 2. Solve exponents. 7 + 25 x 7 - 8
Step 3. Solve multiplication/division. 7 + 175 - 8
Step 4. Solve addition/subtraction. 182 - 8
= 174
(20 votes)
• I don't understand what he says about distributing the number, in this case 6, to 30 and 6.
(4 votes)
• Think of 6x36 as 36 groups of 6, and note that 36 = 30+6.
30 groups of 6 are 30x6 = 180.
6 groups of 6 are 6x6 = 36.
So 6x36 = (6x30)+(6x6) = 180+36 = 216.

The distributive property is very important in arithmetic, algebra, and higher levels of math. Make sure to take the time to understand it well and get more help if you need it. This property says that when we multiply a number by a sum (or difference) of two or more numbers, we can multiply this number by each number in the sum (or difference), then add (or subtract) the products.

Have a blessed, wonderful day!
(20 votes)
• why is it that when a number is out alone ( with no operation) next to parenthesis that you have to multiply
(8 votes)
• The parentheses are one of several accepted symbols that we use to indicate multiplication.
(5 votes)
• why are the parenthesis so important I understand that they tell you what to do first but why?
(8 votes)
• Not only they tell you what to do first, they can also be used to group numbers to make it organized and less confusing.
(9 votes)
• what seven x five
(5 votes)
• Seven x Five = Thirty-five.
5 + 5 + 5 + 5 + 5 + 5 + 5 = 35
(5 / 10 / 15 / 20 / 25 / 30 / 35)
7 + 7 + 7 + 7 + 7 = 35
(10 votes)
• I get the concept but does anyone have helpful hints for remembering 7's so I do not have to skip count / break it down every time like @ ?
(5 votes)
• Switch the numbers. Example:
You know your 5's and 6's,
But you wonder what 7* 5 is.
Then switch it so it says 5* 7,
Then it kinda seems easier.
or
Say you have to do 7* 4,
but you know what 6* 4 is.
Then just add 4.
(7 votes)
• what is 6 times 9
(6 votes)
• 54 is the answer
(2 votes)

Video transcript

We want to figure out how many balloons we have here. And obviously, we could just count these. But now, we have other ways of thinking about it, especially because they're arranged in this nice array, this nice grid pattern here. And the reason why it's useful to not just always have to count it, but to be able to use little multiplication with the number of rows and the number of columns is that you might run into things and you will run into things where it's very hard to count each of the objects individually, but it might be a little bit easier to count the rows and to count the columns. So for example, right over here, we see that we have 1, 2, 3, 4 rows. And we have 1, 2, 3, 4, 5, 6, 7 columns. So you could view this 4 as an array of objects where we have 4 rows. Let me write that down. We have 4 rows and we have 7 columns. And you might already remember that we can calculate the total number of objects by multiplying the rows times the columns-- 4 rows times 7 columns. Now, why does this work? Why will this give us the actual number of objects? Well, we could view this-- we have 4 rows. So we have 4 groups of things. And how many things are in each of those rows? Well, the number of columns. We have 7 things in each of those 4 rows-- so 4 groups of 7. Or you could view it the other way around. You could view that each column is a group. So then you have 7 groups. And how many objects do you have in each? Well, that's what the rows tell you. You have 4 things in each of those columns. And we already know that both of these quantities are going to find the exact same number, the number of things that we have right over here. So these two things are equivalent. 4 times 7 is equal to 7 times 4. And there's a bunch of ways that we can calculate either one of these. We can skip count by 4. We say 4 times 4, 8, 12, 16, 20, 24, 28. And let's see. Let me make sure that that's 7. So 4 times 1, 2, 3, 4, 5, 6, 7-- so we get 28. We could just calculate that there are 28 objects here. And likewise, we could skip count by 7. So we could go 7-- 7 times 2 is 14, times 3 is 21, times 4 is 28, just adding seven every time. And so we could get 28 the other way. Let's do that in that same color. We could get 28 the other way. But if you had a situation where you didn't know-- where you either didn't want to do these techniques or it had been hard to do these techniques or you didn't know what 4 times 7 was off the top of your head, which you should know at some point in the very, very near future. Is there any way to break this down to something that maybe you do know or maybe that's a little bit easier to compute? Well, you could realize that 7 columns is the same thing as 5 columns and then 2 columns. So you could view 7 columns as 5 columns-- so this is 5 columns right over here-- plus 2 columns. So that's just like saying that 4 times 7 is the same thing as 4 times 5 plus 2. And all I do is I replace the seven with a 5 plus 2. 7 has been replaced with a 5 plus 2. Now, why is this interesting? Well, now, I can break this up into two separate arrays. So I could say, well, look, there's the array that has 4 rows and 2 columns right over here. And then there's the array that has 4 rows and 5 columns right over here. So how many objects are in this one, in the yellow one right over here? Well, there's 4 times 5 objects. So there's 4 times 5 objects in the yellow grid or yellow array. And how many in this orange-ish looking thing? Well, there's going to be 4 times 2. And if we take the sum of the 4 times the 5 and the 4 times the 2, what are we going to get? Well, we're going to get the 4 times the 7. We're going to get the 4 times the 5 plus 2. So if we take sum of these things-- and we want to do the multiplication first, so I'll just put a parentheses around that to emphasize that-- this is going to be the same thing as these things up here. And so you might say, oh, well, I know what 4 times 5 is. 4 times 5 is 20. 4 times 2 is 8. 20 plus 8 is 28. And you might say, OK, Sal, I get it. 4 times 7 is 28, which is the same thing as 4 times 5 plus 2. And I see that that's the same thing as 4 times 5 plus 4 times 2. And actually, this is called the distributive property-- that 4 times 5 plus 2 is the same thing as 4 times 5 plus 4 times 2. But I could just do one of these first techniques you talked about. Why is this distributive property that you just showed me useful for computing or doing multiplication problems? Well, let me give you a slightly more difficult one. Let's imagine you wanted to multiply 6 times 36. Actually, I don't need to write that parentheses. So how could you do this? Well, you could decompose 36 into two products or into two numbers where it's easier to find the product of that and 6. So for example 36, is the same thing as 30 plus 6. So this is going to be equal to 6 times 30 plus 6. And what's this going to be? Well, we just saw. 6 times these two things added together first, this is going to be equal to 6 times 30 plus 6 times 6. Notice, we distributed the 6-- 6 times 30 plus 6 times 6. Now, why is this useful? Why was this useful at all? I'm going to put parentheses to emphasize. We're going to do the multiplication first. In general, when you see multiplication and addition in a row like this, or division, you want to do your multiplication and division first, then do your addition and subtraction. So what's 6 times 30? Well, this is easier to calculate. 6 times 3, we know to be 18. So 6 times 30 is going to be 180. And 6 times 6-- well, we know that's going to be 36. So this is going to be 180 plus 36. And what's that going to be? 180 plus 36-- well, 0 plus 6 is 6. 8 plus 3 is 11. 1 plus 1 is 2. So you just figured out that 6 times 36 is equal to 216. And what we just did with the distributive property, this is actually going to be how you're going to multiply all sorts of larger numbers, way larger than what we just saw. So the distributive property, which hopefully you're pretty convinced by, based on how we broke things up, is a super useful thing as you want to compute larger and larger numbers. And you're going to find it even more useful when you go even further in your mathematical career.