Question 936543
Smith purchased 10 dollar stamp, 15 dollar stamps and 20 dollars with a total value of 840 dollars. If the number of 15 dollars stamps is 8 more than 10 dollar stamps and there are 10 more of the 20 dollar stamps than of the 15 dollar stamps, how many of each does she 
receive?
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Equations
10t + 15f + 20w = 840 dollars
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f = t + 8
w = f + 10
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Substitute for t and w and solve for "f"::
10(f-8) + 15f + 20(f+10) = 840
45f + 120 = 840
45f = 720
f = 16 (# of 15 dollar bills)
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t = f-8 = 8(# of ten dollar bills)
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w = f + 10 = 26 (# of twenty dollar bills)
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Cheers,
Stan H.
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