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3rd Grade Math10 chapters | 87 lessons

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Lesson Transcript

Instructor:
*Lynn Knott*

Lynn has taught 1st and 3rd grade and has a master's degree in Educational Leadership.

Missing quantities in math problems can take one or more steps to find. In this lesson, you'll learn how to solve multiplication and division problems with missing quantities and how to complete multi-step problems.

Your teacher tells your class she wants to play a game. She writes three words on the board and tells you to do your best to remember them. The words are ''pencil,'' ''book,'' and ''horse.'' She then tells the class to close their eyes while she erases one of the words. When you open your eyes, you see only two of the words still on the board: pencil and book. The other word has been erased.

Your teacher asks your class for the missing word. Everyone says ''horse.'' How did they know this was the correct answer? By seeing which other words were left on the board!

This is the same way we handle missing quantities in multiplication and division equations. We use the information we have to solve for the missing quantity. A **missing quantity** is just the part of the equation that's not there; it's a mystery number. A missing quantity can be the first, second, or last number in an equation.

**Multiplication** is just adding the same number over and over until you reach the total you want. You may already know 2 x 4 = 8. If we take away one of the numbers, would the problem change? Let's take a look.

The way we solve for a missing quantity in a math problem is to try a number in the missing spot and see if the problem stays the same. If it changes the equation, we try another number.

We could also solve for the missing quantity by dividing. For example: 8 / 2 = 4 and 8 / 4 = 2.

**Division** is just backwards multiplication. We start with a bigger number and break it into pieces. Let's look at the equation 8 / 2 = 4. We know the problem stays the same even if we remove a number.

We solve this problem by looking at the information we already have and try a number we think would work. If it changes the equation, we try another number. We can also solve for the missing quantity by multiplying. For example: 2 x 4 = 8 and 4 x 2 = 8.

A **multi-step problem** is a problem that takes a little extra work to solve. It's a combination of two or more problems within one equation. It usually has parentheses around each part. Let's look at this example:

(2 x 3) + (4 x 1) = _____.

The way we solve this problem is to complete the first equation in parentheses. We know 2 x 3 = 6. We write our answer under the original problem. We then solve the next part of the equation, which is 4 x 1 = 4. Again, write the answer under the original problem. Our new equation combines the answers from both parts, or steps: 6 + 4 = 10.

A **missing quantity**, or the part of the equation that's not there, in a math problem can be found by using the other numbers in an equation. Sometimes these quantities are found in a **multiplication** problem, which is just adding the same number over and over until you reach the total you want, or a **division** problem, which is backwards multiplication. You may have more work to do with **multi-step problems**, which are a combination of two or more problems within one equation, but you still solve them by completing one part at a time.

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3rd Grade Math10 chapters | 87 lessons

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