Lesson+5

media type="file" key="SubtractionMissionLesson5.mp3" width="240" height="20" (Click here to listen to the text of the lesson.)
 * Lesson 5: Mission (Im)Possible… Subtract! **

Agent X, you have done a fantastic job so far! You have acquired several secret weapons, which are important in succeeding in the ultimate challenge. Right now, it is time to combine some of the old and new skills to do some serious subtracting. Good luck!

Even though you are on a mission, which requires the subtraction of a two-digit number form another two-digit number, the main skill required is subtracting one-digit numbers. This is a skill you have already. You just need to make sure you apply it in the correct order and do some more practicing. This should be an easy task for a skilled agent like you. But keep your eyes open – danger lurks around every corner!



** Recall: **

You have already learned two important rules from subtracting a one-digit number from another two-digit number: (1) Always subtract the subtrahend’s digit (number at the bottom) from the minuend’s digit (number at the top). (2) To identify the difference between the two numbers either count up or down.

Let’s look at the following video.

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Which strategy do you prefer? Are you more comfortable with counting up or counting down?

** Process: ** This procedure applies to every subtraction problem, no matter whether you are dealing with one-, two- or more-digit numbers.

The important thing to know when solving problems with more than one-digit numbers is that you start with the digits in the 1s place and then move on to the digits in the 10s place.

So remember that:

(1) You always subtract the bottom number from the top number. (2) You identify the difference by counting up or down. (3) First you subtract the digit in the 1s place. (4) Second you subtract the digit in the 10s place.

Always ask yourself after you have subtracted your digits, whether you have done it in the correct order. (Remember: special agents are very reflective people!)

There are no exceptions to the rule – and it’s so easy to remember because you start with the 1s and 1 is the very first number.

So remember: in subtraction we start with the 1s, just as in counting we start with the number 1.

Can you think of a better way of remember this procedural rule? Write it into your mission notebook.

** Practice: ** And now it’s time to practice! Complete the following subtraction algorithms by writing them into your Mission Notebook.



Compare your results with those on the **Lesson Exercise Keys** page, **Exercise 8**. How did you do? Write down things you want to remember for future subtraction missions – unless you are confident in this task already.

** Conclusion: ** Agent X, you have completed another important task, acquired another secret weapon for your mission. Remember the steps in their procedural order:

(1) Make sure you subtract the subtrahend (the number at the bottom) from the minuend (the number at the top) by counting up or down. (2) First subtract the digit in the 1s place. (3) Second subtract the digit in the 10s place. (4) Reflect whether you have subtracted in the correct order.



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

(1) Complete the following subtraction problems. Show which digit you subtract first and which digit you subtract second.



(2) Read the following two statements and then decide which statement is true and which is false. Mark the statements with **“T”** and **“F”** respectively.

(_) During the subtraction of two-digit numbers, I subtract the digits in the 10s place first.

(_) During the subtraction of two-digit numbers, I subtract the digits in the 1s place first.

(3) Subtract only the digit in the 1s place in the following subtraction problems.



(4) Subtract only the digit in the 10s place in the following subtraction problems.



Go to the **Lesson Exercise Keys** page. Open **Exercise 9** 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 6.

Click here to go to Lesson 6.