Question 1150414
erin has a total of thirty coins worth $4.25. All of the coins are either nickels, dimes, or quarters. Of the number of quarters is the same as the number of dimes, how many of each kind of coin does she have?
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n + d + q = 30
q = d
---> n + 2d = 30 
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5n + 10d + 25q = 425
q = d
---> 5n + 35d = 425
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n + 2d = 30
n + 7d = 85
------------------------ Subtract
-5d = -55
d = q = 11
n = 8
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8*5 + 11*10 + 11*25 = 425
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It can be solved with a single unknown/variable, but so what?
There is no charge for variables.  What is the advantage?