4th Grade Math
Multiplication and Division of Whole Numbers
Unit 2
Lesson One: Using Multiplication and Division
Multiplication
Watch these videos to learn how to multiply large numbers!
Looks harder than it is right? When you are multiplying by a number that is two or more digits, you have to do a different process than you're used to.
Follow along with these videos and practice with them!
Look at the website below for more help with multiplication!
Practice Question: What is 65 x 23?
1,495. If you didn't get this, double check your work and rewatch the videos from the unit! Work along with them!
Division
Division is "repeated subtraction", in other words, the number of times a number can be broken down by another number!
Look at the site below to learn how to do long division! Then watch the videos for more help!
Addition and Subtraction: Price and Time
When making change (doing math with money) there are many things you have to remember! Remember that there are 100 cents in one dollar.
With a certain amount of money, you can only afford a few things. By adding the things you want to buy and subtracting them from the total amount of money you have, you can see how much money you will have left over.
Example: Say you have $16.50 in your pocket. You want one candy bar and one book. The candy bar is $1.55 and the book is $6.45 If you add the things you want and subtract them from the amount of money you have, you can see how much money you will have left over.
So lets do it. $1.55 + $6.45 = $8. The things you are buying cost $8. If you bought these, you would still have $8.50 left to buy other things.
Time can also be added and subtracted.
Remember, there is 60 minutes in an hour and 24 hours in a day. By seeing what time something started and ended, you can see how much time has elapsed.
If Jonah starts eating food at 8:35 and he finishes at 9:20, how long was he eating for? To figure this out, you can first figure out how much time passed between 8:35 and 9:00. At 9:00, 25 minutes have passed. After that, 20 minutes passed until 9:20. 25 + 20 minutes have passed. This means that 45 minutes have passed between 8:35 and 9:20.
Lesson Two: Estimation
Rounding and Estimation
Do you recall these videos? If you need a refresher to remember rounding, watch them again!
When rounding numbers, there is a rule of thumb! When the number is four or less, you round it down. When the number is five or more, you round it up to the next number. A common saying for this is 4 or less, let it rest, 5 or more let it soar!
Let's look at the number 47 and round it to the nearest ten. In order to round it, we need to look at the number before it, in this case the 7 in the ones place. Since it is greater than 5, we must round it up. We round up the number in the tens place. Therefore, it is rounded up to 50. 47 is closer to 50 than it is to 40.
Lets use rounding to solve 21 x 18. First, lets round these numbers to nearest tens. This would make it 20 x 20. When multiplying numbers that end with 0, you multiply it as if there were no zeros, then add the zeros at the end.
When you do 20 x 20, you can start by doing 2 x 2 which gives you 4. There are two zeros when you do 20 x 20. You add these two zeros to the end of the 4. This tells you the best estimated answer, 400.
When you do the actual equation (21 x 18) it gives you 378. This is very close to 400, the estimated answer.
Estimating is taking an educated guess using processes like rounding. Estimating helps you find a close, good answer without having to do a whole equation. In multiplication and division, you can use estimating to get a close answer for an equation.
Lesson Three: Factors and Multiples
Factors are numbers that you can multiply to get a number. For numbers under 100, you can find the factors by making a factor rainbow. This allows you to see the numbers that multiply to make a number. You can start from number 1 and go up. Look at this one on the right that is finding the multiples of 24. You can make one of these on your paper when you're answering a question!
Watch the video below to see how you can make a factor rainbow!
Multiples are numbers that a number can become when multiplied by something. Take this equation. 4 x 5 = 20. 20 is a multiple of 4 and 5.
Let's look at the example on the left and find the factors and multiples.
42 is a multiple of 6 and 7. 6 is a multiple of 2 and 3.
6 and 7 are factors of 42. 2 and 3 are factors of 6. 7 and 1 are factors of 7.
Hint!
If the digits in a number add up to be a multiple of 3, then the number is also a multiple of 3!
For example, 63. If you add the numbers 6 + 3 it gives you 9. 9 is a multiple of 3. This means that 63 is also a multiple of 3!
Lesson Four: Identify Prime and Composite Numbers
Prime Numbers- Prime numbers are numbers that can not be multiplied to get. They have no factors (except for 1) and they are not a multiple of any number. They can only be made by multiplying themselves by 1. Some prime numbers include 2, 3, 5, 7, 11, and 13. There are an infinite amount of prime numbers!
Composite Numbers- Composite numbers are numbers that aren't prime. These are numbers that can be made by multiplying other numbers. They have factors and they are multiples of other numbers.
Hint!
Any number that ends in an even digit is not prime! If it ends with an even number, it can always be divided by 2! 2 is always a factor of a number that ends with an even digit!
For example, 74! It ends with an even number, 4. This means that 2 is a factor. 2 can be multiplied by 37 to make 74! This means 74 is not prime!
Lesson Five: Generating Patterns
Just like there are patterns in shapes and colors, there are patterns in numbers as well. When there is a pattern in numbers, there are rules that you can continue the pattern with.
Take the pattern 0, 3, 6, 9, 12..... What comes after this? To figure this out, you have to figure out the rule of this pattern first. What is a comm on trend in these numbers? What happens every after every number? 3 is added to the number to get the next number. The rule is "add 3".
So what comes after this? To figure this out, apply the rule. Keep addding 3. After 13 would come 15, 18, 21 and so on.