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LESSON PROBLEM: Computation: You want to buy rice cookies for your friends. You need 15 quarters to buy them. But you have only 9 quarters. How do you find out how much more money is needed to buy the snacks for your friends?
STUDY GUIDE
Have you ever tried to figure out how much more you need of something? Or, how much more money you need to buy something, like a rice snack? You want to buy rice cookies for your friends and you need 15 quarters to buy them. You have only 9 quarters. How do you find out how much more money you need to buy the cookies for your friends?
Subtraction is a way to figure out how much more we need of something or how little of something we have. It is important to know both addition facts and subtraction facts. In the question above, you need 15 quarters to buy rice snacks and you have only 9. You want to know how much more you need.
To find out start counting forward until you reach the number you need. It is just like climbing a ladder. You are climbing a ladder with 15 steps or rungs. You are at rung number 9. How many more steps do you need to climb to reach the 15th rung?
Start climbing and as you climb each rung say: 10 (1), 11 (2), 12 (3), 13 (4), 14 (5), 15 (6). How many numbers do you see? The number in parenthesis is the number of steps you climbed. As you climb one step, you reach the 10th rung. When you climbed 6 steps, you reached the 15th rung. You need 6 more quarters to have 15 quarters for the rice snacks.
You invited 12 friends to your birthday party. Three friends have told you they can't come. You now need to plan for less friends. You want to know how many are coming to your party. How do you do it?
You invited 12 friends and 3 are not coming. What do you do? Use the ladder example again. Start at the 12th rung. Now come down 3 rungs. Count backwards and when you have moved three steps backwards you have the answer.
Move one step down to reach rung number 11 (1). Move down one more and reach rung number 10 (2). Move one more rung down to number 9 (3). The answer is: 9 friends will come to your party. Plan a party for 9 friends instead of 12.
The table below helps when subtracting numbers. It gives subtraction facts to 19.
Subtraction Facts to 19
 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
19 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
18 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
17 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
16 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
15 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
14 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
13 13 12 11 10 9 8 7 6 5 4 3 2 1 0
12 12 11 10 9 8 7 6 5 4 3 2 1 0
11 11 10 9 8 7 6 5 4 3 2 1 0
10 10 9 8 7 6 5 4 3 2 1 0
9 9 8 7 6 5 4 3 2 1 0
8 8 7 6 5 4 3 2 1 0
7 7 6 5 4 3 2 1 0
6 6 5 4 3 2 1 0
5 5 4 3 2 1 0
4 4 3 2 1 0
3 3 2 1 0
2 2 1 0
1 1 0
0 0
Now do the above examples using the subtraction table.
In the first example, you needed 15 quarters and you had only 9. Find the larger number in the first column and the smaller number in the first row. Move down the column under 9 and to the right in the row of 15. The row and the column meet at 6. This is your answer.
Now do the second example using the subtraction table. Find 12 in the first column and 3 in the first row. Move down the column under 3 and to the right in the row of 12. The row and the column join at 9. This is the answer.
The order of numbers cannot be changed like in an addition table. Subtraction is a oneway business. So subtract the row from the column.
Do the following using the subtraction table. Try to learn the table. It will help you when solving many subtraction problems and will help you work faster.
Rice Cookies Needed  Cookies in Stock Need to Buy
12  6
8  5
16  7
16  9
15  7
14  0
12  5
11  9
10  4
19  17
Now you are ready for more subtraction problems.
ACTIVITIES
Go to a grocery store. Write the prices of various snacks and the different varieties of rice. Find out which costs the most and by how much. Show your work.
When subtracting the prices, only look at the dollar amount. Don't use the amount of cents if it is less than 50 cents. If the cents amount to 50 or more than 50, consider it as one dollar.
For example, if a snack is priced at $2.79, count it as $3, but if it is priced at $2.49, count it as $2.
EXTENDED LEARNING
You have $20. You go to a store and buy rice snacks that cost $12.57. How much money do you get back?
Don't round off the amount of cents. Find the exact amount to check if the money you received back is correct or not.
Make some problems of your own and have a friend find the answers.
VOCABULARY
 climb
 ladder
 rung
 row
 column
 subtraction
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