Joe has a collection of nickels and dimes that is worth $4.20. If the number of dimes were doubled and the number of nickels were increased by 7, the value of the coins would be $6.65. How many dimes does he have?
d=number of dimes, n=number of nickels
second equation: 20d + 5n =665-35=630
from first: 10d = 420 - 5n
substitute in second: 2*(420-5n)+5n=630
210=5n => n=42
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