Be careful to distinguish between the value of the items and the quantity of the items. A table is useful for distinguishing between quantity and value in this type of word problems.
Example 1:
John has 50 stamps, some worth 15¢ and some worth 20¢. If their value is $9.50, how many of each kind does John have?
Solution:
Step 1: Set up a table with quantity and value.

quantity 
value 
total 
15¢ stamps 

20¢ stamps 

together 
Step 2: Fill in the table with information from the question.
John has 50 stamps, some worth 15¢ and some worth 20¢. If their value is $9.50, how many of each kind does John have?
Let  x =  number of 15¢ stamps 
y =  number of 20¢ stamps  
Total =  quantity × value 

quantity 
value 
total 
15¢ stamps 
x 
15¢ 
15x 
20¢ stamps 
y 
20¢ 
20y 
together 
50 
950 
Step 3: Add down each column to get the equations
x + y = 50 (equation 1)
15x + 20y = 950 (equation 2)
Use Substitution Method
Isolate variable x in equation 1
x = 50 – y (equation 3)
Substitute equation 3 into equation 2
15(50 – y) + 20y = 950
750 – 15y + 20y = 950
20y – 15y = 950 – 750
5y = 200
y = 40
Substitute y = 40 into equation 1
x + 40 = 50
x = 10
Answer: John has ten 15¢ stamps and forty 20¢ stamps.
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