Lesson+4

media type="file" key="SubtractionMissionLesson4.mp3" width="240" height="20" (Click here to listen to the text of the lesson.)
 * Lesson 4: Trick or trade! **

Have you ever been in a situation in which you badly wanted or needed something (maybe a toy, a book, a treat) but didn’t have the money to pay for it? Has anyone ever offered you to trade this toy or treat for something else? Well, then you already know what this lesson will be about. Special agents are often confronted with situations, which require trading. When on a subtraction mission, knowing how to apply the Trade-First Method is another secret weapon every special agent desperately needs. During this lesson you will learn how to apply the Trade-First Method. There is no way a special agent can complete a subtraction mission without this secret weapon. Work hard to be successful when you meet the ultimate challenge that awaits you at the end of the mission!



** Recall: ** You have already learned the simple path in solving a subtraction problem when subtracting a one-digit number from another one-digit number.

Look at another example:



Counting up from 3: 4, 5, 6, 7 => **4** Counting down from 7: 6, 5, 4, 3 => **4**

As learned in the previous lesson, you apply the same path when subtracting two-digit numbers as long as the subtrahend’s digit in the 1s place is not larger than the minuend’s digit in the 1s place.



** Process: ** Now we are going to look at the method you need to apply when coming across a subtrahend’s digit larger than that of the minuend: the Trade-First Method.

There are two major activities to be completed in the Trade-First Method: (1) Trading from the 10s place. (2) Adding to the 1s place.

Let’s look at the following video.

media type="youtube" key="m4pq-PlhFX8" height="349" width="560"

Did you follow the video carefully to identify all the steps? If not, watch it a second time and then compare it with the following procedure of steps:

(1) Locate the minuend’s digit in the 10s place. (2) Trade one 10 for ten 1s. (3) Subtract 1 from the minuend’s digit in the 10s place. (4) Locate the minuend’s digit in the 1s place. (5) Add ten 1s to the minuend’s digit.

To help you check whether you have applied the procedure correctly, always cross out the old numbers (in the 10s and in the 1s place) and write down the new numbers as in the following example:



Can you think of a strategy that will help you to remember the steps? Here is an example: Think of the two agents in the video: The “small guy” (i.e. the 1s digit) is trading/getting 10 from the “big guy” (i.e. the 10s digit). So, the big guy is helping out the small guy. Does this help?

** Practice: ** Let’s try it with some examples. Complete the examples by writing them into your mission notebook. (You don’t have to complete the entire subtraction, just the trading process.)

Example:

Now check whether you have traded correctly by going to the **Lesson Exercise Keys** page, **Exercise 6**. How did you do? In case you are not yet confident, write in a sentence what you need to keep in mind for working on future subtraction problems.

** Conclusion: **

Agent X, you have completed one of the most important tasks of this mission. The secret weapon you acquired, the Trade-First Method, will allow you to solve any subtraction problem. Remember the two major steps in their procedural order:

(1) Trade from the 10s place. (2) Add to the 1s place. (The big guy helping out the small guy!)



** Assessment: **

Go ahead and earn another star on your special agent badge by completing the following assessment.

(1) Arrange the following steps for “trading” in the correct order. Write down the respective number in front of each statement.


 * Trade one 10 for ten 1s.
 * Add 10 to the minuend’s digit in the 1s place.
 * Subtract 1 from the minuend’s digit in the 10s place.
 * Go to the minuend’s digit in the 10s place.
 * Go to the minuend’s digit in the 1s place.
 * Cross out the minuend’s digit in the 10s place.
 * Write down the minuend’s new digit.

(2) Now explain the method of trading in your own words. Use the following example in your explanation: 75 – 37 =


 * Number problem || Explanation of Trading ||

(3) Demonstrate trading in the 10s place in the following subtraction algorithms.

Example:

(4) Demonstrate the addition of one 10 to the 1s place to complete the process of trading in the following subtraction algorithms.

Example:

Go to the **Lesson Exercise Keys** page. Open **Exercise 7** and compare your answers. If you have all answers correct, add another star to your badge in the Mission Notebook. If you need more practice, please repeat the lesson tomorrow before you work on Lesson 5.

Click here to go to Lesson 5.