4th Grade - Math

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 with you. What do you do in order to find out how much more money you need to buy snacks for your friends?


STUDY GUIDE

Have you ever tried to figure out how much you are short of something?  Or how much more money you need to get something -- say a rice snack? 
You want to buy rice cookies for your friends, and you need 15 quarters to buy them.  But you have only 9 quarters with you.  How do you find out how much more money you need to buy snacks for your friends?

Subtraction is a way to figure out how much we are short of something, or how much more we need of something.

It is important to know both addition facts and subtraction facts.  Today let's explore subtraction facts. You need 15 quarters to buy rice snacks, and you have only 9.  You want to know how much more you need.  

To find this you 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 have to climb to reach the 15th rung?  

Start climbing, and as you climb each rung you say 10 (1), 11 (2), 12 (3), 13 (4), 14 (5), 15 (6).  How many numbers do you see here?  The number in parenthesis is the number of steps that you climb.  As you climb one more step, you reach the 10th rung.  When you have climbed 6 steps, you will reach the 15th rung.  

You need 6 more quarters to collect 15 quarters for the rice snack.

 You had invited 12 friends to your birthday party.  But 3 of your friends have told you they can't come.  So now you have to arrange for a smaller number of children.  But you want to know how many.  How do you do it?

You had invited 12 friends, and now 3 are not coming.  What do you do?  Use the ladder example again. Start at the 12th rung. 
No come down 3 rungs.  Count backwards and when you have moved three steps backwards, that is the answer.

Move one step down to reach rung number 11 (1), then down one more and reach rung number 10 (2), and then one more rung down to number 9 (3).  The answer is: 9 friends will come to your party.  Arrange a party for 9 friends instead of 12.

The table below is helpful in subtracting numbers.  It gives you 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 1514 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 as it can be done for addition.  Subtraction is a one-way business.  So subtract the row from the column. 

Try to do the following exercise using the subtraction table.  Try to learn the table.  It will be useful in solving many subtraction problems, and 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 can play a subtraction game.



ACTIVITIES

Go to a food store. Note down the prices of various snacks and of different varieties of rice. Find out which costs the most and by how much.

While subtracting the amounts, only look at the dollar amount. You need not bother about the cents if they are less than 50 cents. In case the cent amount is 50 or more than 50, consider it as full one dollar.

For example, if a snack is priced at $2.79, count it as 3 dollars, but if it is priced at $2.49, count it as 2 dollars.




EXTENDED LEARNING

If you go to a store, and buy a number of rice snacks costing you $12.57, how much money would you get back if you gave a 20 dollar bill to the cashier?

You are not to round off the cent amount. You have to find the exact amount to check if the money that you received back is correct or not.




VOCABULARY
  • climb
  • ladder
  • rung
  • row
  • column
  • subtraction


Click here to play Rice Rampage!
How is Rice Grown:
Farming rice is hard work. Click here to learn more about the Stages of Farming Rice.
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