SOLUTION: An account executive bought 300 stamps for $73.80. The purchase included 15 cent stamps, 20 cent stamps, and 40 cent stamps. The number of 20 cent stamps is four times the number o

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Question 201432: An account executive bought 300 stamps for $73.80. The purchase included 15 cent stamps, 20 cent stamps, and 40 cent stamps. The number of 20 cent stamps is four times the number of 15 cent stamps. How many 40 cent stamps were purchased?
Answer by Alan3354(69443) About Me  (Show Source):
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An account executive bought 300 stamps for $73.80. The purchase included 15 cent stamps, 20 cent stamps, and 40 cent stamps. The number of 20 cent stamps is four times the number of 15 cent stamps. How many 40 cent stamps were purchased?
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Number of each:
f = 15
t = 20
y = 40
15f + 20t + 40y = 7380
f + t + y = 300
t = 4f
15f + 80f + 40y = 7380
95f + 40y = 7380
19f + 8y = 1476
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f+t+y = 300
f+4f+y = 300
5f + y = 300
y = 300-5f
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19f + 8y = 1476
19f + 8*(300-5f) = 1476
19f + 2400 - 40f = 1476
-21f = -924
f = 44
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t = 176
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y = 80