Lesson+2

media type="file" key="SubtractionMissionLesson2.mp3" width="240" height="20" (Click here to listen to the text of the lesson.)
 * Lesson 2: Preparing for Mission (Im)Possible **

A key element to every mission’s success is the right preparation. An unprepared agent is bound to fail or run into serious difficulties, even danger. Make sure that you learn all the important preparation steps to succeed in subtraction. There is no time for trial-and-error here. Knowing the right steps will become one of your secret weapons.

During this lesson, you will recall the main steps of the subtraction process (i.e. preparing the digits for subtraction and subtracting digits in the 1s place before digits in the 10s place) and practice the procedure to stack and vertically align digits in preparation for the actual subtraction process.

One of the key elements to succeed in two-digit subtraction is to stack and vertically align the digits correctly. Lesson 2 will help you to review and practice this. Pay close attention and work hard!



** Recall: ** You are already a master in subtracting one-digit numbers. I am sure you remember that you need to stack digits before beginning with the subtraction process. Do you remember how to stack the digits? Usually there is always one bigger number and one smaller number. Which one is at the top? Let’s look at the following examples.



Isn’t it funny, that the bigger number is at the top? Well, at first it seems strange but then it makes much sense: if the number at the top would be smaller, we wouldn’t have enough digits for subtraction, right?

** Process: ** So you have already learned that numbers need to be stacked before subtraction. You have also learned that when stacking the numbers, the bigger one goes to the top. This is the same in all subtraction problems, no matter whether you are having one- or two-digit numbers. The difference with two-digit numbers is that you have another step to consider: aligning the digits vertically. Let’s look at the following video about stacking and aligning numbers.

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So, the important thing to remember is that the bigger number is at the top, that the digits in the 1s place are in one column and the digits in the 10s are in another column.

Let’s recall the steps you need to take in preparing your numbers for subtraction: (1) **Stack** the numbers vertically with the biggest at the top. (2) **Align** the numbers vertically with the 1s under the 1s and the 10s under the 10s. (3) After you have prepared your digits for subtraction, **check** that you have done it correctly and check again whether the bigger number is at the top and the numbers are aligned correctly according to their place value.

I am sure you noticed that some of the verbs in the steps are bolded, i.e. **stack, align, check**. Keep these keywords in mind to easily remember the procedure. (Remember, this is one of your secret weapons!)

** And now, let’s practice! ** Prepare the following numbers for subtraction. Write them in your Mission Notebook. (a) 69 – 54 = (b) 75 - 67 = (c) 18 - 9 = (d) 84 – 63 = (e) 47 - 12 =

Done? Then check whether you have prepared the numbers correctly by going to the **Lesson Exercise Keys** page, **Exercise 2**.

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.

** Summary: ** Agent X, you have completed an important first session and acquired an important secret weapon for your mission. Remember the steps in their procedural order: (1) Stack (2) Align (3) Check.



** Assessment: ** And now, go ahead and earn the second star on your special agent badge by completing the following assessment.

(1) Prepare the following numbers for subtraction (i.e. stack and align).

(a) Take away fifty-seven from sixty-nine (b) 85 – 47 = (c) 26 – 18 =

(2) Write the following steps for subtraction in the correct order in your Mission Notebook.


 * Compare the digits in the 1s place and decide what path to take.
 * Verify that the number problem is a subtraction problem.
 * Apply Trade-First Method if applicable.
 * Verify the result.
 * Stack and vertically align digits.
 * Subtract the digits in the 10s place.
 * Subtract the digits in the 1s place.

(3) Which of the following subtraction problems are correctly stacked? Write them in your Mission Notebook.



(4) Which of the following subtraction problems are correctly vertically aligned? Write them in your Mission Notebook.



Go to the **Lesson Exercise Keys** page. Open **Exercise 3** and compare your answers. If you have all answers correct, add the second star to your badge in the Mission Notebook. If you need more practice, please repeat the lesson tomorrow before you work on Lesson 3.

Click here to go to Lesson 3.