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LESSON PROBLEM: Computation: Suppose you have 7 rice cookies. You give 3 to your sister. If you want to know the number of rice cookies you will have left, how would you do it?
Would you add the two numbers or do something else?
STUDY GUIDE
You have 5 rice cookies. You give 1 to your younger brother. To find out how many you have left, count backwards.
Count 1 backwards
You have five cookies. You have given one to your younger brother. So you have one less. To count backwards, you count 5, 4, 3, 2, 1, etc. By going back from one number to the next, you are reducing one each time. 4 is one less than 5, 3 is one less than 4, 2 is one less than 3 and so on.
5
4
3
2
1
After giving one cookie to your brother, you are left with 4 cookies. When you give one more, you are left with 3. When you go on giving one at a time, you are left with 2 and then with 1 and finally with none. All five of the cookies are gone.
You count backwards one when you give one cookie away. Now what number comes before:
2
5
6
3
7
9
10
8
1
4
Counting Backwards Two at a Time
You have 9 rice candies and you want to give 2 to your sister. Now count two numbers backwards. You will be left with 7 candies. We skip 8 and come to 7. If you want to give 2 to your brother as well, you count back 2 twice. Now you come to 5. Look at counting backwards by two's:
10, 8, 6, 4, 2, 0
9, 7, 5, 3, 1,
If you start with 10 and count backwards by two's, you get the numbers 10, 8, 6, 4, 2 and 0. And if you start with 9 and count backwards by two's, you get the numbers 9, 7, 5, 3 and 1. This stops at one. You cannot give two cookies to someone when you have only one.
Subtracting is taking away or reducing. The sign used for subtraction is ''. It is read as minus. The numbers are written as 7  2. You read it as 7 minus 2.
This can be written horizontally as 7  2 or vertically as
7
 2
Either way, you work the problem the same way. Count two backwards to get 5 as your answer.
Now do the following:
6 8 9 10 7 5 3 4
 2  2  2  2  2  2  2  2
Counting Backwards Three at a Time
To subtract 3 from a number, count 3 backwards. You have 9 rice cookies and you give 3 to your friend. To find the number of cookies you have left with, count 3 backwards. Count 8, 7 and 6. After giving 3 cookies to your friend you will have 6 cookies left.
Now do the following:
6 8 9 10 7 5 3 4
 3  3  3  3  3  3  3  3
ACTIVITIES
Draw ten pieces of rice candy on a piece of paper. Color them and then cut them out.
You have a box containing 10 rice candies. Choose some to give to your Mom. How many candies are left? Check your answer by actually counting the candies left. Mix the number of candies that your Mom takes. How many are left? Do this two more times.
Now, using a different number of candies, how many candies are left after giving some away. Check your answer by actually counting the candies left. Mix the candies that you give away to see how many are left each time.
Do this four more times, using a different number of candies each time.
EXTENDED LEARNING
If you were asked to subtract 7 from 10, how would you do it? Would you use backward counting? Explain why or why not.
VOCABULARY
 assume
 backwards
 reduce
 skip
 twice
 horizontally
 vertically
 certain
 actually
 remain
 difference
 risotto
 recipe
 survive
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