Question 516802
x = number of five-cent stamps
5x = value of the five-cent stamps in cents
y = number of twenty-cent stamps
20y = value of the twenty-cent stamps in cents
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5x +20y = 700 cents
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three times as many 5-cent stamps as 20-cent stamps
x = 3*y
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substitute
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5(3y) +20y = 700
15y + 20y = 700
35y = 700
y = 20
.
x =3y = 3(20) =60
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check the value
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5(60) = 300 cents
20(20) = 400 cents
300+400 = 700 cents
correct
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Answer:  He bought 20 twenty-cent stamps and 60 five-cent stamps.