Question 163600
    Warren has 40 coins (all nickels, dimes, and quarters) worth $4.05. He has 7 more nickels than dimes. How many quarters does Warren have?
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Let d = times
then from: "He has 7 more nickels than dimes"
d+7 = nickels
and from: "Warren has 40 coins"
40-d -(d+7) = 33-2d = quarters
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Since, he has a total of 4.05:
4.05 = .10d + .05(d+7) + .25(33-2d)
4.05 = .10d + .05d + .35 + 8.25 - .50d
4.05 = 8.60 - .35d
.35d = 4.55 
d = 13 (number of dimes)
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quarters:
33-2d = 33-2(13) = 33-26 = 7 (number of quarters)