Lesson+3

media type="file" key="SubtractionMissionLesson3.mp3" width="240" height="20" (Click here to listen to the text of the lesson.)
 * Lesson 3: Reaching a crossroad – which path to take? **

The most challenging point in each mission is to take the right decision when reaching a crossroad. Heading down the wrong path will not only be a waste of time but also jeopardize the entire mission. To be able to determine which path is the right one for you to take, you will need another secret weapon. This lesson will provide you with the weapon but it will be up to your skills as a thinker and inquirer to uncover it. Good luck!



** Recall: ** Do you remember how to solve a subtraction problem with one-digit numbers? I am sure you do: you count either up or down.



Counting up from 4: 5, 6, 7, 8, 9 => **5** Counting down from 9: 8, 7, 6, 5, 4 => **5**

Now try the same with the following two-digit numbers by first subtracting the number on the **right** (the digit in the 1s place) and then the number on the **left** (the digit in the 10s place). Write it into your Mission Notebook.



Now try the same with the following subtraction problem:



 Why does it not work with the second example? Are you able to figure it out?

** YES! **

It doesn’t work this way because you can’t take away 6 from 4!

** Process: **

The principle that applies here is:

** If the subtrahend’s digit in the 1s place is larger than the minuend’s digit in the 1s place, then the Trade-First Method has to be applied. **

** WHY? ** Because you can’t subtract a larger number from a smaller number! Makes sense, right?

Therefore, in order to chose the right path, compare the numbers in the 1s place. Decide which one is larger. If it is the bottom number (subtrahend’s), then the path to choose will be the Trade-First Method.

Maybe this sentence will make remembering it easier: ** “If the subtrahend’s digit is large, you cannot choose the simple path.” **

You could also try to keep in mind that both “small” (i.e. the smaller subtrahend’s digit) and “simple” (i.e. the simple path for subtraction) both begin with “s”.

** Practice: ** (1) Read the principle out loud three times:

** If the subtrahend’s digit in the 1s place is larger than the minuend’s digit in the 1s place, then the Trade-First Method has to be applied. **

(2) Close your eyes and try to say it again. You can peek if it doesn’t work.

(3) Try to say it in your own words.

(4) Look at the following examples. Write the subtraction problems of “Set 1” in your mission notebook. Circle the digits in the 1s place. Compare the digits in the 1s place. Write either “**WT**” or “**NT**” below the example. (“**WT**” means with trading and “**NT**” means no trading or the simple path).

** Set 1 **



** Set 2 **



Compare your answer with those for **Exercise 4** on the **Lesson Exercise Keys** page.

How did you do? If you identified the correct path in all examples and feel confident, move on. If not, work on Set 2.

** Conclusion: ** Well done, Agent X! You have worked hard. Let’s look again at the principle in graphic format.



And remember your **secret weapon**: ** “If the subtrahend’s digit is large, you cannot chose the simple path.” ** This will guarantee success when you get to the ultimate challenge.



** Assessment: ** Please complete the following assessment in your Mission Notebook to earn another star for your special agent badge.

(1) Look at the following subtraction problems. Decide which path they require and circle the correct answer, either **WT** (“with trading”) or **NT** (“no trading”).



(2) Look at the following subtraction problems. Mark those in which the subtrahend’s digit in the 1s place is larger than that of the minuend.



(3) Write the following two statements into your Mission Notebook and fill the gaps with the words **“larger”** and **“smaller”**:

(a) If the subtrahend’s digit in the 1s place is --- than that of the minuend, I can select the simple path to solve the subtraction.

(b) If the subtrahend’s digit in the 1s place is --- than that of the minuend, I have to apply trading to solve the subtraction.

Go to the **Lesson Exercise Keys** page. Open **Exercise 5** 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 4.

** Click here to go to Lesson 4. **