3rd Grade - Solving Problems Using Relationships

 
     
 
     
 
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3rd
Problem Solving
Solving Problems Using Relationships
Find the total cost of multiple items, given the cost per unit.
Algebra and Functions: solving problems between two quantities The ability to calculate how much a group of identical items costs if the price of one of them is known.
 

Sample Problems

(1)

If one toy car costs $2.00, how much do 3 cars cost? ($6.00)

(2)

If a pound of candy costs $8.25, how much will you pay for 2 pounds? ($16.50)

(3)

If the price of oranges is 2 oranges for $.75, how much will 6 oranges cost? ($2.25)

(4)

If a Playstation game costs $25.83 how much will 2 games cost? ($51.66)

(5)

If dog food costs $3.63 a box and your dog can eat 3 boxes in a week how much will it cost to feed your dog each week? ($10.89)

Learning Tips

(1)

Encourage your child to help you estimate costs when you are selecting multiples of a particular item.

(2)

Remember that if your child is still learning multiplication that another way to reach the solution is to add together the price per item as many times as there are items, but this calculation usually takes longer and is prone to errors because of the number of calculations that must be done.

(3)

Encourage your child to consider the calculations he or she has made to see if they are reasonable. If dollar toys are being purchases and there are only 10 of them, an answer in the hundreds of dollars is not reasonable and is enough off the mark that it should seem unreasonable to a child as well.

(4)

If many items are being purchased and the price per item involves many digits as well, allow your child to figure (or check) the answer on the calculator. The set up for the problem (knowing what to multiply) is the same so there is reinforcement for mastering the skill; only the calculation practice will be missing.

(5)

Encourage your child to explore the reverse of this process (knowing the total cost, and how many items were bought, how much was each item?) because this concept is the root of understanding unit pricing (cost per ounce or pound or item) that is the base for comparison-shopping.

Extra Help Problems

(1)

Complete this chart:


Cost per item

Number of Items

Total Cost

1

$1.25

3

($3.75)

2

$ .99

7

($6.93)

3

$ 2.15

2

($4.30)

4

$ 6.29

1

($6.29)

5

$ 9.00

80

($720.00)

6

$1.10

4

($4.40)

7

$3,99

8

($31.92)

8

$ 9

12

($108)

9

$6.35

6

($38.10)

10

$ 2.30

7

($16.10)

11

$10.00

2

($20.00)

12

$ ,60

9

($5.40)

13

10 cents

19

($1.90)

14

$15.01

2

($30.02)

15

$ .80

2

($1.60)

16

$ 2.99

8

($23.92)

17

$ 4

2

($8)

18

$8.22

8

($65.76)

19

$ 9.24

3

($27.72)

20

Fifteen cents

9

($4.50)

21

Fifty cents

60

($30)

22

$6.78

3

($20.34)

23

$1.00

7

($7)

24

$1.96

2

($3.92)

25

$8.55

1

($8.55)

 

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